For your information, if you enable Firefox VPN you must disable DNS over HTTPS otherwise it does not work, you get the same error as the Internet is down (which is not).
This is another of those situations where I really really don’t get why some obvious issue that is going to annoy million users doesn’t get addressed.
All it takes is to check if DOH is enabled whenever the VPN gets engaged, then you either inform the user she/he needs to disable DOH or you do that automatically, then ideally revert to DOH when the VPN gets disengaged.
Edit: it looks like it has been fixed with Firefox 152.