Firefox CPU usage

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This article describes how to prevent or work around high CPU usage in Firefox.

Only on specific pages


If you're only having problems with sites that use Flash and updating to the latest Flash version doesn't help, you can use the Flashblock extension to selectively enable or disable it [1][2]

If the problem happens when viewing PDF files in the browser, it's likely a problem with the Adobe Reader plugin. One possible fix is to create a "WWW_OpenURL" directory in the "AdobeUM" directory. [3] [4] Another workaround is to disable the browser plugin so that PDF files open in Adobe Reader as an external application, as explained here.


Some sites can cause high CPU usage through extensive use of JavaScript. You can disable JavaScript selectively using the NoScript extension.

Report the site

Report the site using Help -> Report Broken Web Site. Reporting the problem will help identify sites with problems so Mozilla evangelists can help fix their problems, and also help Mozilla developers track down any bugs in Firefox that could be causing excessive CPU usage.

All the time

After Standby or Hibernate

There are some bugs in older versions of Firefox and Flash that can cause Firefox to consume lots of CPU after the computer has been in standby or hibernate mode. These can be fixed by updating to the latest Flash player and either Firefox or Firefox 3.


Problematic extensions can cause excessive CPU usage.

Windows Compatibility mode

Firefox can cause excessive CPU usage if it's set to run in compatibility mode for an older version of Windows.[5] To fix the problem, right-click on the Firefox icon you click to start Firefox, click Properties, click the Compatibility tab, make sure the check box under Compatibility mode is unchecked, and restart Firefox.

Hardware acceleration

Disabling cursor and bitmap hardware acceleration on some video cards and drivers can reduce CPU usage when displaying images.[6] To check hardware acceleration on Windows, right click on the desk top to get the display properties menu. Then select Settings > Advanced > Troubleshoot. This article shows how. For more detailed information on troubleshooting display properties, see this article.

Windows prefetch

Deleting the Windows XP or Vista prefetch files for Firefox (C:\Windows\Prefetch\FIREFOX*) can prevent problems with the jpeg_fdct_islow thread consuming lots of CPU.[7]

See also

External links