Some recent additions to my Firefox arsenal:

Aging Tabs - Tabs begin to "grey out" if you haven't focused them in awhile (or based on a time interval if you like.) Good for knowing which tabs to close if the tab bar is getting crowded.
Fission - The progress bar is shown in the URL bar, a la Safari on the Mac.
FoxyProxy - Better than SwitchProxy if you ask me. Lets you use proxies automatically based on patterns, etc.
Resizeable Form Fields - As the title suggests, lets you arbitrarily resize form fields. Idea probably stolen from the upcoming feature in Safari.
Resurrect Pages - If a page is dead, gives you easy access to caches (Coral IDN, Google Cache, etc.)

And, for those who use Youtube and have Greasemonkey installed, I highly recommend the YousableTube script, which makes YouTube a lot better, including allowing you to customize the size of the video window (defaults to fill the browser), gives you a link to download the video in FLV format, etc.