You need to completely uninstall as in when you uninstall do not keep anything, that won't be enough though to really remove everything you need to go into the programs folder and delete the entire mozilla folder, don't forget to check in the roaming folder (you can access it by using run and typing in %appdata%) once you get there push the up arrow to go back a folder and you should see local, locallow and roaming, check each one for anything related to firefox or mozilla and delete it.
And once you do that you should restart to clear any stuff in the temp folder and you should have truly uninstalled firefox completely, then you can reinstall.