Try changing DNS over HTTPS (DoH) to "Off" or "Max Protection" and in Connection Settings change "Use system proxy settings" to "No proxy".

Does it still happen in Troubleshoot Mode?

You can check the connection settings and check for possible issues with a Proxy or VPN service.

• 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 also:

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

You can do a malware scan, just to be sure.

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware

Actually, the problem resolved itself for reasons unknown... I had rebooted multiple times, resetting Malwarebytes, etc. It somehow fixed itself when I logged into the only website that worked (Mozilla). Not sure why.