Matrix bridges have nothing to do with encryption, they read the messages exactly the same way a client would, and send them to the other side of the bridge exactly the same way a client would.

It’s only for the free tier, and it’s removable, just a little annoying cause you have to do it every time you write a mail.

Yes but 90% of the people using Firefox won’t bother or notice. I already struggle trying to make all my relatives switch to Firefox, I can’t imagine getting them to download a specific version or build it themselves…