As of Firefox 53 in March 2017, WinXP and Vista won't be supported any longer. http://www.ghacks.net/2016/09/27/firefox-53-no-support-for-windows-xp-or-vista/ https://support.mozilla.org/t5/Install-and-Update/Important-Firefox-is-ending-support-for-Windows-XP-and-Vista/ta-p/31270 You can install the Firefox ESR 52 version which will get security updates into early 2018, to get "more time" using Firefox. [https://www.mozilla.org/en-US/firefox/organizations/ Firefox; Extended Support Release] {web link} [https://www.mozilla.org/en-US/firefox/organizations/notes/ ESR Notes] [https://www.mozilla.org/en-US/firefox/organizations/system-requirements/ System Requirements]

See: *https://support.mozilla.org/en-US/kb/your-hardware-no-longer-supported * https://www.mozilla.org/en-US/firefox/49.0/releasenotes/ [FX:49]:Ended Firefox for Windows support for SSE processors You can use the last Firefox 45.9.0 ESR version that is more recent than the Firefox 48 release. Firefox 45.9.0 ESR (see the Show selector): *https://www.mozilla.org/en-US/firefox/organizations/all/ *https://www.mozilla.org/en-US/firefox/organizations/notes/ *https://www.mozilla.org/en-US/firefox/organizations/system-requirements/