The only add-ons I have are Webex, SQLite, and Startpage, so no ad blockers. But some web sites seem to have blank spaces where you'd expect ads to be so maybe there's ad-blocking built into FireFox? If so how do I reconfigure it? I don't think it can be something AVG is doing because that would affect Edge as well.

More info: Meetup thinks I'm not logged in; when I try to log in to Meetup it says "e-mail is required" and "password is required" although I've filled them both in. If I click on "Log in with Facebook" instead and give permission via Facebook's pop-up I then get "Bad Request" and "Your browser sent a request that this server could not understand. Size of a request header field exceeds server limit."