DO NOT  remove personal data when you uninstall your current Firefox version, because all profile folders will be removed and you lose personal data like bookmarks and passwords from profiles of other Firefox versions. 

   It is important to delete the Firefox program folder to remove all the files and make sure that there are no problems with files that were leftover after uninstalling.
   http://kb.mozillazine.org/Uninstalling_Firefox 

Do a clean reinstall, delete the Firefox program folder before (re)installing a fresh copy of the current Firefox release. Download a fresh Firefox copy and save the file to the desktop. https://www.mozilla.org/en-US/firefox/new/ Uninstall your current Firefox version, to cleanup the Windows registry and settings in security software. ''' DO NOT ''' remove personal data when you uninstall your current Firefox version, because all profile folders will be removed and you lose personal data like bookmarks and passwords from profiles of other Firefox versions. It is important to delete the Firefox program folder to remove all the files and make sure that there are no problems with files that were leftover after uninstalling. http://kb.mozillazine.org/Uninstalling_Firefox Your bookmarks and other personal data are stored in the Firefox profile folder and won't be affected by an uninstall and (re)install, but make sure that "remove personal data" is NOT selected when you uninstall Firefox. If you keep having problems then also create a new profile.