you have a few options:
using synaptic you can reinstall firefox.
using synaptic you can remove firefox and then install firefox.
look/wait for an update to firefox to be released.
I haven't seen any recent posts about firefox errors on 7.04 fiesty.
it maybe gremlins and a reboot later it all works.
The extreme fix is to reinstall mint and not hit the upgrade to 7.04 button but just update packages.