Are you able to clear cache and cookies? Does the same thing happen in a Private Window?

Do you use a Proxy or VPN?

You can check the connection settings.

• Settings -> General -> Network: Connection -> Settings

If you do not need to use a proxy to connect to internet, select "No Proxy" if "Use the system proxy settings" or one of the others do not work properly.

See "Firefox connection settings":

• https://support.mozilla.org/en-US/kb/websites-dont-load-troubleshoot-and-fix-errors

Does it still happen in Troubleshoot Mode?