I saw my same issue described in an archived thread "Several users have reported connection issues with Google sites including YouTube. This has been linked to the "zero round trip resumption" feature. That is intended to speed up website connections, but there seems to be some kind of compatibility issue. Currently, there is only a hidden way to disable this and see whether it helps". I tried the method given (going to about:config and turning security.tls.enable_0rtt_data to false) but it did not work. I tried uninstalling and reinstalling, but I accidentally dragged the Firefox in the installer directly onto my dock (on Mac) and that Firefox was working perfectly fine. But I noticed there was "Firefox" drive running on my system. To fix it, I realized I had to drag the Firefox in the installer into my applications folder and run that Firefox. So I did so, and ejected the Firefox drive. Using the new Firefox that was actually installed in my Applications, I am now stuck with the same problem of Google sites not being accessible.