I've been using Firefox for awhile now and thought I'd post the extensions I use.
- Bloglines Toolkit - If you use Bloglines for your news aggregator, then this extension is indispensible. It's got the notifier as well as context menu subscription options.
- Resize The Search Box - Why Firefox doesn't ship with this I'll never know but this one's golden. Allows you to increase the size of the search box. Read the comments to see how to get it working properly.
- Web Devloper - Anybody who does any development of HTML pages should find this extension just what the doctor ordered. Allows you to make live edits to the CSS of pages, gives you plentiful validation options, and much more. I used to have a bunch of bookmarklets that did some of this but the Web Developer extension did away with them all.
If you are running Firefox, what extensions do you have installed? If you aren't running Firefox, why not?