I am usually not that big a fan of add-ons, plugins etc., for the simple reason that I really hate having to collect them from different websites only to get the program I want running and having to repeat all that every time i switch to another computer. But for some plugins the gain in productivity outweighs the extra effort downloading them. For Thunderbird the add-ons I can recommend are:
- Enigmail, which gives you gnupg integration for Thunderbird
- External Editor, which allows you to compose mails with your favorite editor (think (g)vim).
- Colored Diffs, which is a must-have if you read mailinglists where patches are posted.
The last plugin is especially cool, since it allows you to read patches much easier than in the raw format.