Since the version in 10.04 Mozilla made some big changes
in the FF libraries, where settings are located and in encrypting data.
Your best bet is to rename the old folder and re-install a fresh version It
will be a hidden folder called .mozilla. You will need to show hidden
folders to rename it. Ctrl+H should do that. Then go to the folder and
rename it something like mozilla.old with no dot at the front. That way you
can save your bookmarks. If they are already backed up then you can just
delete the .mozilla folder before you re-install. Deleting the folder
itself may be enough to get FF to work as it will re-build the folder from
scratch.