I understand and agree with you in principle but "Tell the mail service to change their application" is such a long-shot step to fix the issue that I simply want to be able to use my browser to access my webmail...
From where I'm standing it's easier to switch to hardened Firefox than to switch mail providers just to continue to use Librewolf
EDIT: I would love an explanation of wtf the WebCrypto API even is and why you think it's spyware, though
EDIT 2: Proton claims the WebCrypto API is a browser security feature. It's also developed by Mozilla
EDIT 3: Fixed this by simply forcing an update to the latest version of LibreWolf. Not a browser problem, not a proton mail problem, WebCrypto API is not spyware.