Genius Thunderbird plugin that sets a custom From address based on the recipient address of the email. Genius, exactly what I've been seeking for years. Now to get it setup...
A handy addon by one of the Greasemonkey developers that allows ctrl-t to open a new tab to the right of the current tab. It has some quirks though...
Builds of Google Gears for 64-bit firefox.
FoxClocks lets you keep an eye on the time around the world - or just your local time - by putting small clocks in your statusbar. Hat tip James Farmer.
HackBar gives you a really big address bar with a load of extra functions to test sql injections and so on. Useful for testing. Not currently 3.0 friendly.