Plugin support in Thunderbird
From MozillaZine Knowledge Base
Starting with Thunderbird 52 all NPAPI plug-ins are disabled. 
Plugin support was added in Thunderbird 3.0, mainly for multimedia support. It is a obsolete feature nowadays as the market has moved to HTML5 and more web-centric APIs. 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 such as Flash, not extensions and themes. It was disabled in 3.1.6 due to performance issues caused when plugins were loaded. Thunderbird 3.1.* supported a plugin.disable setting. That seems to have been a temporary workaround that no longer exists. 
Thunderbird 5.0 enabled plugins by default if it wasn't a email account, though that has only been verified for 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.
Thunderbird supports its own copy of the same plug-in scanning settings as Firefox. Thunderbird will scan for plug-ins on startup. If it discovers one it will load it and your only option is whether to enable or disable it. Set plugin.scan.plid.all false using the Config Editor to prevent it scanning the directories specified in the Windows registry for PLIDs (a string in URI format that identifies a plugin uniquely). See plug-in scanning for how to prevent it scanning some other directories for plug-ins. If you have problems identifying plugins that are loaded 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.