I don't have any extensions, but I do like some of the firefox hacks. I think everyone knows about the hacks (and the pitfalls of some of them...) but here's a site that served me well enough. I think that I was able to locate some of the problems in linked forums, etc. from this page, as well as some decent hacks.
BTW, WHY are these extensions so great...don't just name, expound for the rest of our benefit.
In that light, I like some of these "hacks" because they've sped things up, and helped with firefox crash issues...I have had the most trouble with opening .PDF files, but I think some of the # of request hacks might have helped. Then again, I'm probably telling you the wrong hack is responsible for allowing me to open .PDF's with fewer problems.
But I digress...
http://www.hackaday.com/entry/1234000803024910/
Thanks,
Hokiesandwich