So if nothing else has been working for you, then try the following:
- Open about:config (you can do this by typing in about:config in the url bar for ThunderBrowse or via Tools < Options < Advanced < General < Config Editor)
- In the filter bar, type plugin
- Verify the following settings are set to these values:
- plugin.scan.plid.all to true
- plugin.disable to false
- mailnews.message_display.allow.plugins to true
- Restart Thunderbird.
This should get everything working. I don’t think I have this set on my main dev machine, and I’ve been having problems with plugins but have been too busy to investigate it. I’m pretty sure I haven’t changed this pref on my main machine, which would mean this might be a bug that was fixed in Thunderbird.
Either way, that should do it. I’ve updated the settings code so it will modify these preferences as well.