Look inside the .mozilla/firefox directory of your home folder. Is anything missing?
You could also potentially solve the problem by renaming the above directory, which would force Firefox to recreate a profile for you using default settings. You'd lose all of your bookmarks and things, but if this isn't a big problem for you, it could be the best solution. You would do:
Also, if you upgraded from Ubuntu 7.10 to 8.04, then you went from Firefox 2 to Firefox 3 beta. Your settings should have been transferred transparently, but maybe something went wrong. You can install Firefox 2 through Synaptic (it won't interfere with Firefox 3) and see if FF 2 works properly.
mv ~/.mozilla ~/.mozilla-old