In my /home/username/.mozilla/x8hbj9lp.default folder, there's another folder called "extensions." I would assume this is where the extensions live.
Do you have one like that?
If so, I'd back up that folder, export your bookmarks, then trash your .mozilla folder. Open up Firefox. Close it. Then put the extensions folder back and import your bookmarks.
If that doesn't work... well, extensions are typically what make Firefox unstable. That happened on my wife's Powerbook. Firefox kept crashing for certain sites, so we got rid of her profile folder and rebuilt Firefox, installing one extension at a time and putting in only the essential ones.
Sometimes, it's that extension developers do not upgrade their extensions. Sometimes extensions are not compatible with each other.