Try completely removing using Synaptic (right-click should have this as an
option) or use *apt-get --purge remove firefox*
*
*
The problem is if there is a bad package (and I do not know that is the
case) removing and re-installing does not work because it will not download
a fresh copy but will re-install the bum one from the cache. You need to
remove the application and delete the package and get a fresh one. I am not
saying this will resolve the problem just that re-installing is not always
a solution.