See: * http://kb.mozillazine.org/Browser_will_not_start_up#XULRunner_error_after_an_update The updater wasn't able to update all the files and some were left as older versions. You need to do a clean reinstall. Download a fresh Firefox copy and save the file to the desktop. * Firefox 5.0.x: http://www.mozilla.com/en-US/firefox/all.html * Uninstall your current Firefox version. * Do not remove personal data when you uninstall the current version. Remove the Firefox program folder before installing that newly downloaded 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. 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. ---- * http://kb.mozillazine.org/Profile_folder_-_Firefox * http://kb.mozillazine.org/Profile_backup