This has been blogged about in the usual places already, but for those who haven't seen it:
There's an addon for Firefox that defragments its sqlite database, called Vacuum Places. I recently gave this a whirl and was pleased with the results. The responsiveness of my address bar was noticeably improved. Now, bear in mind that I use Firefox a lot. I have over 4000 bookmarks -- its kinda getting out of control. Those who don't use it as much probably won't notice much of a difference, but give it a whirl and see what it does for you. Note that there is a disclaimer saying to backup you profile (easily done with MozBackup), but I haven't had any issues with it.
Some other addons you might want to check out:
Adblock Plus - never see another ad
SkipScreen - auto waits on file upload sites
DownloadHelper - download youtube videos, a page of images at once, and more
DownThemAll! - download everything on a page, based on filters (all pdfs, all images, etc)
Stylish - loads custom css into sites (get some here)
Tab Mix Plus - one of the biggest reasons I use it is for the 'duplicate tab' option, but the 'close other tabs', 'close right tabs', and 'close left tabs' are pretty nifty too.