First, you should do a back up if you haven't. Then try going into:
Finder>home folder>Library>Preferences
Move the org.Mozilla.firefox.plist to the desk top.
Restart the computer and open firefox.
If it works, delete the old plist. If not, put the old plist back. Try the step that was mentioned above about deleting the profile folder. You can copy it to an external hard drive if you would rather. And that would be better then just deleting it.
If none of this works, delete everything off your computer that has the words mozilla or firefox in them. Then reinstall firefox. If that doesn't work, backup and reinstall the os. If that doesn't work, do with out firefox, go back to leopard, or sell your mac and buy a windows machine. Because if none of this works, the last thing you're gonna wanna see is a mac.