This browser extension supports Firefox for Linux, if you have any issue running Firefox Proton VPN extension on Linux, I recommend you reach the customer support: https://protonvpn.com/support-form
About the protocol, it's simply HTTPS because that's what browser provides to do a proxy with encryption. So if browsing websites works, then it means the protocol is not blocked. However the port used might be blocked. Maybe you can try to enable Secure Core in the options as it uses the port 443 (which cannot be blocked neither as it would make the browser unusable).
I also take note of the requested profiles feature.
