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/en-US/kb/end-support-windows-xp-and-vista?cache=no https://support.mozilla.org/en-US/kb/install-older-version-of-firefox?cache=no Be sure to read everything here. If you still want to downgrade, look under; I still want to downgrade. Click the Directory of other versions and languages link. Look for the directory of the version that you want. But, remember that old versions may have security issues. You should also check out [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]

It is possible that security software (anti-virus, firewall) is causing the problem. Try to disable security software temporarily to see if that makes a difference. Make sure to download the ESR version from this website: *https://www.mozilla.org/en-US/firefox/organizations/all/ Maybe consider to use an external download manager if you keep have issues with using Firefox.