Download a fresh copy of Firefox ( http://www.mozilla.com/firefox/all.html ) and do a reinstall of Firefox over the top of the current version. If that still causes the same problem then do a clean reinstall. Uninstall your current Firefox version and remove the Firefox program folder before installing a fresh copy of the Firefox installer. 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. You can skip the step to create a new profile, that is not necessary for this issue. ..... See http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall

It worked! Thanks!