I have the VPN problem too. For me, it's a proxy thing. My company puts an HTTP proxy in-place inside of their VPN, and Firefox picks up the settings from MacOS.

When I disconnect from VPN, Firefox still has the VPN's proxy settings. I have to restart it to refresh Firefox's cached proxy settings. So that means I effectively have to restart Firefox any time I change between VPN and non-VPN.

If that's your problem too, then I don't have a fix for it. I'd love to hear if anybody else does though.