This article lists some extensions that are known to cause problems. There are hundreds of extensions, and most problems are not listed here. If you do not find the problem listed below, follow the steps outlined in the Standard diagnostic - Firefox article.
Extensions are not written by Mozilla and they are not tested with the browser or in combination with each other. Extensions are normally installed by visiting a website such as addons.mozilla.org and clicking an "Install" link but there are also many applications that include Firefox extensions or toolbars that can be installed without you being aware of it. , 
Updating extensions can solve many problems. Extensions installed as experimental do not get automatically updated. Extension versions needed after upgrading Firefox may be experimental or only available on the extension developer's site.
Most extensions can be disabled or uninstalled in Firefox using the Add-ons manager ("Tools -> Add-ons -> Extensions") although a few extensions may present removal problems. In some cases, you may need to run Firefox in a special mode called "Firefox Safe Mode" to access the Add-ons manager. You may also need to reset preference changes made by the extension that may persist after it is disabled or uninstalled. See Uninstalling add-ons and Uninstalling toolbars for more information.
|Extension||Extension version affected||Problem||Workaround||References|
|A9 Toolbar||All||Causes Unresponsive Script Warning||Uninstall A9 Toolbar|||
|Adblock||All||Having the OBJ_TABS setting enabled may interfere with the Java, Flash or Windows Media Player plugins. Content requiring these plugins may not display unless this setting is disabled.||If updating to Adblock 0.5.3.042 or later does not resolve the issue, keep the Adblock OBJ_TABS setting disabled.||  |
|Causes Google Gmail Chat to crash.||?|||
|Adblock Plus||All ?||Can interfere with Flash content, most often on Mac OS.||Uncheck "Show Tabs on Flash and Java" in Adblock Plus options.|| |
|0.7.5.2||Windows error opening Internet shortcut or local HTML file - Firefox||Update to Adblock Plus version 0.7.5.3 or later|||
|All-In-One Gestures||All versions using Firefox 2?||May cause Firefox crashes. Also conflicts reported with spellchecker in Linux Mac OSX versions.|||
|All-In-One Sidebar||0.6.4 and prior||New extensions will not install. Also occurs in version 0.6.4 if updated from an earlier version or if used together with the SessionSaver extension.||Uninstall, then (re)install version 0.6.4. Disable opening the Extensions window in the sidebar (Misc. -> Extensions -> Open in Sidebar).|||
|Amazing Media Browser||0.17||The "Block all embedded objects" preference is set to "Yes" by default, causing embedded media normally handled by plugins to disappear, including Flash content. Also interferes with the IE Tab extension.||Change the "Block all embedded objects" preference setting to "No".|||
| Ask.com Toolbar
Also available as:
Ask Toolbar for Firefox. May also be bundled with other software.
|All?||May cause redirected searches, failed keyword searches and/or the "File not found" error, Firefox can't find the file at jar:file:///C:/Program Files/Mozilla Firefox/chrome/en-US.jar!/locale/browser-region/region.properties||Disable or uninstall in Firefox via "Tools -> Add-ons -> Extensions". May need to uninstall via Windows Control Panel (see Uninstalling toolbars). Delete the file "AskSearch.js" (if found) after uninstalling. May also need to reset preferences related to Ask.com (such as keyword.URL) via about:config.||        |
| AVG Safe Search
Installed with AVG Anti-Virus (LinkScanner component).
|8.0||Blocks use of ENTER key from the location bar.||Disable in Firefox "Tools -> Add-ons -> Extensions" or update to AVG 8.5 or above.||  |
|All?||Crashes, high CPU usage, cache is purged after restarting; Firefox remains in the processes list at exit causing "already running but is not responding" error when restarting Firefox.||Disable in Firefox "Tools -> Add-ons -> Extensions" or remove the extension by reinstalling AVG without the LinkScanner component (see AVG faq# 1338).|| |
|Cooliris (formerly PicLens)||All?||Firefox may remain in the processes list at exit, causing a "Firefox is already running but is not responding" error (or problems with bookmarks, history, and toolbar buttons due to a locked Places database) when restarting Firefox.||Disable or uninstall the extension. Note: A conflict with the anti-phishing component of Norton 360 2.0 or Norton Internet Security 2008.5, if installed, can cause the hang at exit and locked Places database. As a workaround, disable Phishing Protection in Norton's settings or remove the Norton toolbar from Firefox as detailed here.||  |
|Copernic Desktop Search Home Connector||Prior to 3.2.1||Excessive Firefox CPU usage and/or Firefox hangs at exit||Update to version 3.2.1 or above.||  |
|DownThemAll||1.0|| May cause web browsing to slow to a halt to the point that nothing loads.
Important: DownThemAll 1.0 changes certain "network.http" preferences that will need to be reset, even if you disable or uninstall the extension.
|Update to version 1.0.1 and reset these preferences via about:config:
||  |
|All (?) since 1.0||May cause Windows error opening Internet shortcut or local HTML file - Firefox||If updating DownThemAll to version 1.0.2 or later does not resolve the issue, disable or uninstall the extension.|||
|EBay Browser Highlighter||?||Installs with Skype. Highlights things like products, phone numbers, important people. Causes Firefox to take up to 30 seconds to load.||Disable or uninstall the extension. ||  dev-apps-firefox|
|eBay Hot Toolbar||?||Highlighted text follows cursor as you move within a web page||Disable or uninstall the extension.|||
|external IP||Prior to 0.9.9.3||Causes Firefox crashes and authentication popups asking username/password for "Development Area" at http://www.crazzy.se||Update to version 0.9.9.3 or above|| |
|Fast Dial||2.16||Adds "spam" links to the Bookmarks menu, adds the UserLogos search plugin and makes it the default search engine, searches may redirect.||Update to version 2.18 or above (available from the Fast Dial Homepage). If necessary, delete unwanted bookmarks and change to your preferred search engine in the Firefox Search Bar.||  |
|2.15||Conflicts with the IE Tab extension (broken/blank Location Bar) and the Tab Mix Plus extension (oversized tab bar) , adds "spam" links to the Bookmarks menu, adds the UserLogos search plugin and makes it the default search engine, searches may redirect. May interfere with toolbars from other extensions.||Update to version 2.18 or above (available from the Fast Dial Homepage). If necessary, delete unwanted bookmarks, reset toolbar customizations and change to your preferred search engine in the Firefox Search Bar.||  |
|Fasterfox||1.0.x||Hangs or slows down Firefox||Disable prefetching, select "Optimized" preset instead of "Turbo".|||
|All||Changes made by Fasterfox can interfere with Web site access and persist after it is uninstalled.||Reset options to defaults in Fasterfox before uninstalling it; otherwise, reset preferences in Firefox to the defaults (or create a new profile).|||
|Filterset.G Updater||Prior to 0.3.0.2||Memory leak when used in conjunction with FlashGot||Update to version 0.3.0.2.|||
|Firebug||All?||May cause excessive CPU usage; Firefox may hang when a pop-up window is accessed.||Disable Firebug when you don't need to use it||  |
|1.5.4 +?||Downloaded files will be stored in memory, possibly causing high memory use.||Disable Firebug when you don't need to use it|||
|Prior to 1.3.2||Firefox crashes on logout from Yahoo! Mail, reported in Firefox 2. Note: Firebug 1.3.2 (and above) requires Firefox 3.||Disable Firebug when you don't need to use it.|||
|Firefox Google Bookmarks||Prior to 0.2.1?||Interferes with spell checking in Firefox version 2.||Update to version 0.2.1. If that does not work, uninstall extension.|||
|FireFTP||All?||Can cause excessive CPU usage with Firefox 22.214.171.124||Update Firefox to the latest version|||
|FlashGot||Prior to 0.5.9.995||Memory leak when used in conjunction with Filterset.G Updater||Update to FlashGot version 0.5.9.995 or later|||
|ForecastFox||version 0.9.x and all satellite images versions?||Memory leak||Update or uninstall and install Forecastfox l10n. There are unconfirmed reports that there are also memory problems with versions 0.9.3 and 0.9.4.|| |
|Prior to 0.9.5.1||Causes excessive CPU usage with Firefox 2.0 and Firefox 126.96.36.199||Update to Firefox 188.8.131.52 and ForecastFox 0.9.5.1|||
| Free Download Manager
|Prior to 1.3.4||The Free Download Manager application (FDM) installs an extension for Firefox integration which can cause Firefox crashes when right-clicking a Web page.||Download and install the latest version (FDM 2.5 build 758 will update the extension to v 1.3.4) . Note: The FDM extension (v 1.0 - 1.3.1) is on the Add-ons Blocklist; v 1.3.2 is not blocklisted but it also crashes Firefox.||  |
|Google Browser Sync||All?||Causes excessive CPU usage in general or with GIF animations||Uninstall|||
|?||May cause notably increased disk usage (in gigabytes) by creating large log files. May cause periodic Firefox freezes.||Uninstall|||
| Google Toolbar
Also available from the "official" Google Toolbar home page
Google toolbars are also bundled with other software
|All?||"Send with GMail" option overrides default mail client. If the Google Toolbar is uninstalled, mail links may stop working in Firefox||If still installed, UNcheck "Send with GMail" in Google Toolbar Options. If uninstalled, use about:config to reset the preference network.protocol-handler.external.mailto to true (double-click will toggle).|| |
|3.x (beta?)||Windows error opening Internet shortcut or local HTML file - Firefox||Disable or uninstall|| |
|3.1||New tab page option creates a thumbnail for each visit greatly increases size of places.sqlite||Right click the Google logo in the Google toolbar > Options > Uncheck “Enable the Google new tab page”|||
|All||Causes excessive memory use and other performance problems.||Disable when not needed.|
|Hebrew Calendar||Prior to 184.108.40.206||Causes "ReferenceError: JEvent is not defined"||Update to a newer version|||
|IE Tab||All||Memory leak||Uninstall IE Tab and install IE View Lite|||
|iMacros||Prior to 0.5.0.4||Causes excessive CPU usage||Update to a newer version|||
|ImgLikeOpera||0.6.15||"Refresh images in tab on tab select" option causes excessive CPU usage on tab change and erratic text box behavior (i.e. losing caret after selection in google search text box).||Turn off "Refresh images in tab on tab select" option.|||
|Linky||2.6||Gray bar below the status bar with red arrow.||Uninstall Linky.|||
|LiveHTTPHeaders||All?||Uploading a file locks the file so it cannot be renamed, moved, or deleted.||Uninstall LiveHTTPHeaders.|||
|McAfee SiteAdvisor||?||"onLookupFailed 55 1" alerts.||Disable McAfee SiteAdvisor.|| |
|22||Firefox may fail to open after installing or updating the SiteAdvisor extension, which also may be installed as part of a McAfee software suite. This SiteAdvisor support article describes the problem as a conflict between SiteAdvisor and automatic proxy configuration scripts in Firefox.||Either follow the workaround in the SiteAdvisor support article or end the firefox process that remains in memory, start Firefox in Safe Mode and uninstall via Tools -> Extensions. If SiteAdvisor is a McAfee suite component it can also be removed using the McAfee uninstaller.|| |
|23||Causes Firefox to crash with an error in Jar50.dll after installing or updating the SiteAdvisor extension or McAfee component.||Uninstall the SiteAdvisor extension or, if SiteAdvisor is installed as part of a McAfee software suite, remove the SiteAdvisor component using the McAfee uninstaller.|||
|3.3||After bookmarking a page with Bookmark Star can't Edit the bookmark from the Bookmark star nor with Ctrl+D to modify or remove the bookmark in at least Firefox 3.6.12 and 3.6.13. In Firefox beta 4.0b9pre though it doesn't interfere with bookmark star it hangs Google Search and so does not do its function of reporting on sites in search results listing.||Uninstall or Disable the SiteAdvisor extension or, if SiteAdvisor is installed as part of a McAfee software suite, remove the SiteAdvisor component using the McAfee uninstaller.||    |
|Media Player Connectivity||All?||Can cause Unresponsive Script Warning||Uninstall Media Player Connectivity|||
|MegaUpload Toolbar||All?||Causes the error message, Trying to load a non-local URI. Also changes the user agent by adding "MEGAUPLOAD 1.0" which causes some websites to tell you to update to a modern browser.||Uninstall the toolbar. To restore the user agent, type about:config in the Location Bar and filter for general.useragent. If the general.useragent.extra.firefox preference shows a user set status, right-click it and select Reset.|| |
|Middle mouse button no longer opens new links from web pages||Update to the latest version; if the issue persists, disable the extension||  |
|Mouseless Browsing||All?||Can cause excessive CPU usage||Uninstall Mouseless Browsing|||
|Nightly Tester Tools||All||Allows incompatible extensions and themes to run; these incompatible add-ons can cause problems||Update your extensions and themes to those compatible with your version of Firefox|||
|All since 220.127.116.11||Flash and other plugin content is blocked by default on untrusted sites, even content from trusted sites (e.g. YouTube.com) if embedded on an untrusted site.||Uncheck the option to forbid Flash or other plugins in NoScript's Options\Plugins pane or temporarily allow the content by clicking on its placeholder.||  |
|All since 1.1.0||Java is blocked by default on untrusted sites||Uncheck "Forbid Java" in NoScript's Options\Plugins pane (Options\Advanced\Untrusted in earlier versions). In version 1.1.3 and later you can temporarily allow Java content by clicking on its placeholder.|| |
|Prior to 18.104.22.168||Cannot log into eBay or other sites with NoScript "Automatic Secure Cookie Management" enabled.||Update NoScript to the latest version.|||
|"Numbered Links"||All||FormSpy (aka FireSpy) malware identifies itself as "Numbered Links". The real Numbered Links extension is harmless. See this forum topic for more information.||Uninstall the extension. Scan your system for viruses, particularly "FormSpy" and "Downloader-AXM".|||
|OneClick YouTube Downloader||1.0.2 (All?)||Can prevent YouTube videos from playing and Firefox may stop responding at exit||Disable or uninstall this extension.|||
|onFocusReload||?||"Alert! Object Event" message.||Uninstall onFocusReload.|||
|PicLens||See above entry for Cooliris (formerly PicLens)|
|ProCon||Prior to 1.5.2||Replaces all words on webpages with asterisks (******) if set to "censure" words. Fixed in version 1.5.2.||Uninstall ProCon, restart Firefox, reset ProCon preferences in about:config, restart Firefox, reinstall.|||
| RealPlayer Browser Record Plugin
This extension is installed as an option when RealPlayer is installed.
|1.0||May cause various issues including problems with form submission, embedded Flash, or images not loading after a refresh or revisit to the page.||Try updating RealPlayer, which will update the extension, or disable in Firefox via "Tools -> Add-ons -> Extensions". Note: the Uninstall button is "grayed-out", as this extension is installed via the Windows Registry. To remove from Firefox, open RealPlayer, go to Tools -> Preferences -> Download & Recording and uncheck "Enable Web Download & Recording".||    |
|?||May cause the Firefox 3 "Bookmark this Page" dialog to appear when clicking the Bookmarks menu item.||Updating to Firefox 3.0.11 or above should resolve the issue. If the problem persists, disable the extension or remove it in RealPlayer preferences (see above)|| |
|SearchStatus||All?||Causes Firefox to hang for short periods.||Uninstall SearchStatus|||
| Skype Extension for Firefox
This extension is installed as an option when Skype is installed.
|?||Firefox may start with error messages and may run slowly. Google maps or other images may not load. GMail reply fails and may show "Unable to load the rich text editor". Direct links to images or PDF files may cause the error, The image cannot be displayed because it contains errors. Web pages may continually refresh or stop loading. Firefox may stop responding or hang at exit, resulting in an "already running but is not responding" error (or problems with bookmarks, history, and toolbar buttons due to a locked Places database) when you next restart Firefox.||Note: Extension versions prior to 22.214.171.12471 are now blocklisted.
Update Skype to the latest version. You can also disable or uninstall the Skype extension via "Tools -> Add-ons -> Extensions" (the extension is not needed to use the Skype application).
Note: If Norton 360 2.0 or Norton Internet Security 2008.5 is installed, a conflict with Norton's anti-phishing component can cause the hang at exit and locked Places database. As a workaround, either disable Phishing Protection in Norton or remove the Norton toolbar from Firefox (details here).
|      |
|SwitchProxy Tool||All||Causes new Firefox windows to freeze for a few seconds||Disable SwitchProxy's auto-update feature|||
|Tabbrowser Extensions (TBE)||All||General slowdown, conflicts with other extensions, can cause Firefox to not work at all||Uninstall TBE, use Tab Mix or Tab Mix Plus instead|||
|Tabbrowser Preferences (TBP)||All (for Firefox in some languages)|| Gray bar below the status bar at the bottom of the screen which reads:
<button id="catTabbedButton" label="&tabprefs.label;"
|126.96.36.199|| Bar at the bottom of the screen which reads:
|Uninstall TBP and install it directly from the author's website|| |
|ThinkVantage Password Manager Extension||2.0||May cause Firefox to freeze for up to a few seconds when a new window is opened.||Disable or uninstall the ThinkVantage Password Manager extension in Firefox via Tools -> Add-ons -> Extensions. (Note: The Uninstall option may be greyed out, if the extension was installed via a Windows Registry entry).|
|2.0|| May cause a "Failure in chrome registration" error when starting Firefox.
Note: In Windows Vista, exiting Firefox, right clicking the Firefox icon and selecting "Run as administrator" may resolve the issue; if not, use the workaround.
|Disable or uninstall the extension (from Firefox Safe Mode if necessary) if you don't need it. Alternately, find the file tvtpwm_moz_xpi.xpi (usually in "C:\Program Files\Lenovo\Client Security Solution") and open it in Firefox to reinstall the extension.|| |
|2.0||Firefox crashes when opening a pop-up window. Caused by the latest Lenovo Client Security update.||Disable the extension or reinstall it by opening the file tvtpwm_moz_xpi.xpi in Firefox (see above)||  |
|Torbutton||Prior to 1.2.5||New windows opened from links (e.g. popups) are blank, possibly due to a conflict with the Google Toolbar.||Update to the latest version of Torbutton|| |
|Prior to 1.2.4||All bookmarks are missing after upgrading to Firefox 3.6||Update to the latest version of Torbutton|| |
|Prior to 1.2.1||Interferes with cookie deletion. May also change Firefox privacy and security settings or cause performance issues.||Check Torbutton options or update Torbutton to the latest version (or uninstall Torbutton) then recheck Firefox settings. You may need to delete the file "cookies-nontor.xml" to resolve the cookies issue. For other issues, repair or replace the Firefox profile, if necessary.||  |
|Update Notifier||?||A prompt for a username and password from http://mozdev.elliptic.fr/ asking "Oui?"||Uninstall Update Notifier|||
|User Agent Switcher||Prior to 0.6.4||Reinstallation or update causes Firefox to hang||Uninstall previous version in Safe Mode before installing new version|| |
|VeryDelicious||All||Software update screen on opening windows and downloading files.||Uninstall VeryDelicious|||
|Web Analytics Solution Profiler/Debugger (WASP)||Prior to 1.0.9||On Mac OS, installing this extension sets the ~/Library/Caches/TemporaryItems folder to non-writable, causing Download Error -228 and inability to install add-ons.||Open the /Library/Caches/ folder for your Mac user account, context click (right-click) on the "TemporaryItems" folder, choose "Get Info" and, under "Sharing and Permissions", change permissions to "Read & Write".|| |
|Web Developer||1.1.8||excessive CPU usage proportional to how many tabs are active; with 50 tabs and no activity, 58% cpu usage when enabled, 8% when extension disabled.||Disable this extension, and only enable when you need it|
|Wizz RSS News Reader||2.0.0||Update causes Firefox to hang||Uninstall previous version in Safe Mode then install the updated version.|||
|Woot Watcher||1.0.1 (all?)||Periodic Firefox hangs, slowdowns, or excessive CPU usage, possibly related to Woot Off events.||Disable or uninstall the Woot Watcher extension|||
|XPL LinkScanner||2.6||Causes Firefox to crash when using Yahoo, Google, or MSN Search. This extension may be installed in Firefox as part of "System Suite 8" by AvanQuest.||Disable or uninstall "XPL LinkScanner Firefox 2.6" via Tools -> Add-ons -> Extensions.|||
|Yahoo! Toolbar||All?||Causes excessive CPU usage||Uninstall via "Tools -> Add-ons -> Extensions" or use these instructions.|||
|Prior to 1.6.6||Right-click disabled in Firefox 3.5||Update to version 188.8.131.5290220|||
|ZoneAlarm SpyBlocker Toolbar||See above entry for Ask.com Toolbar|