This issue can be caused by the presence of leftover files in the Firefox program folder (defaults\pref) like a file firefox.js that overrides the update URL with a wrong link. See my post here and do a clean reinstall and be sure to remove the Firefox program folder. * [/questions/826858] ---- * 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 [http://kb.mozillazine.org/Profile_folder_-_Firefox 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.