Download a fresh Firefox copy and save the file to the desktop.

   Firefox 4.0.x: http://www.mozilla.com/en-US/firefox/all.html 

   Uninstall your current Firefox version and remove the Firefox program folder before installing that copy of the Firefox installer.
   Do not remove personal data if you uninstall the current version.

   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. 

taken from https://support.mozilla.com/en-US/questions/826858: ''This issue can be caused by the presence of leftover files in the Firefox program folder (defaults\pref) like a file all.js or firefox.js'' ''Download a fresh Firefox copy and save the file to the desktop. Firefox 4.0.x: http://www.mozilla.com/en-US/firefox/all.html '' ''Uninstall your current Firefox version and remove the Firefox program folder before installing that copy of the Firefox installer.'' ''Do not remove personal data if you uninstall the current version. 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.'' ''Your bookmarks and other profile data are stored elsewhere in the Firefox Profile Folder and won't be affected by a reinstall, but make sure that you do not select to remove personal data if you uninstall Firefox.'' ''If the problem persists See also: https://support.mozilla.com/en-US/questions/815130?page=2#answer-185385'' Check and tell if its working.