Try disabling the Google Update plugin.

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 very 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.