Plugin support in Thunderbird
Plugin support was added in Thunderbird 3.0. Supposedly it supported Java, Flash, PDF and Windows Media Player though there was no official list. "Plugins" is sometimes treated as synonymous with add-ons but in this case it means browser type plugins, not extensions and themes. It was disabled in 3.1.6 due to performance issues caused when plugins were loaded.
If you want to disable specific plugins the Firefox plugin scanning settings also exist in Thunderbird. If you have problems getting the right plugins try setting plugin.expose_full_path true and either set about:plugins as your mail start page in Tools -> Options -> General -> Thunderbird start page or install the ViewAbout add-on.
Supposedly in Thunderbird 3.3 plugins will be enabled in non-email situations by default. For example, it will allow them in RSS feeds. Some users have reported YouTube videos would play in Thunderbird 3.3 alpha four builds while they wouldn't play in 3.1. Its not clear yet whether this is due to YouTube playing HTML5 video using WebM, and support for WebM being added in Gecko 2.0 (which Thunderbird 3.3 uses) or not.
- Investigate unnecessary plugin directory scans while loading messages, causes pauses/slow display during mail navigation or click bug report.
- Blocklist url updates for Thunderbird - Change blocklist more information url to https and support specifying application range for plugins in blocklist.xml bug report.
- Embedded (CID) Flash Movie in HTML email is not shown bug report
- Double clicking on PDF email attachment does not open PDF file bug report
- Cannot install plugins (such as Adobe Flash Player) bug report
- PLuginChecker add-on checks if there are any plugin installed without user permission.