You appear to be using Mountain Lion OS X 10.8 and that is no longer supported by Apple and only supported to a limited extent by Firefox. You will be able to use the Firefox ESR and it will have critical security fixes back ported to it until January 2017. * [[Firefox no longer works with Mac OS X 10.5]] * '''[[Firefox support has ended for Mac OS X 10.6, 10.7 and 10.8]]''' I would have hoped if you visit download sites with a working Firefox it would not prompt you to try to upgrade to an incorrect version of Firefox. Please use * '''Download''': https://www.mozilla.org/firefox/organizations/all/ * see https://www.mozilla.org/en-US/firefox/45.0/system-requirements/