hello, normally such issues are caused by a firewall/security software which doesn't recognize & therefore blocks new firefox versions. please remove all program rules for firefox from your firewall and let it detect the new version of the browser again.

Fix problems connecting to websites after updating Firefox

IQ140 said

After updating, pages won't load. Will not connect to Comcast. Restarted computer from scratch, reset modem and router -- nothing works. Tried Safe Mode start, did not work. Internet Explorer works fine. Mail works fine. There were no error messages when the update was loaded. This is a terrible "update"!!!!!!!!! How do I fix?

UpDate: I indeed found firewall was the issue (first time with Firefox updates). I had tried uninstalling and reinstalling but that did not work either. Then I opened the Norton security app window (looking for firewall information). I did not actually change or click on anything in Norton. I decided to try installing again. This time when I hit "Run" instead of "Save", the little Norton window popped up saying the download is safe. Then it installed and opened just fine.

Perhaps this is a quirk of Norton? Or choosing Run instead of Save (and running after saving)?