Then that isn't a Firefox browser or Mozilla VPN issue then.

I don't use Mozilla VPN. Firefox browser works fine unless my ExpressVPN is activated. The error message when, for example, I attempt to access the Hague Permanent Bureau website (hcch.net) is:

Hmm. We’re having trouble finding that site. We can’t connect to the server at www.hcch.net If you entered the right address, you can:

   Try again later
   Check your network connection
   Check that Firefox has permission to access the web (you might be connected but behind a firewall)

When I disconnected Macafe, Windows prompted me to turn on Windows Defender, which I did. Could that be the issue? Sorry, I'm a lawyer, not very tech savy

Then you need to contact ExpressVPN support to fix that issue.