Not every site will accept your Proton address. You could generate a unique email mask for most sites and apps instead of giving them your real email address, wherever your inbox may be. Firefox Relay just forwards the emails it receives to whatever address you give it.
If your goal is to avoid reverse email searches, this is how’d I’d avoid giving my real email address out during signups.
Edit: I think Google won’t accept a proton address, but I may be wrong