i would like this too. i think you can do it with apple email relay.
currently i have example.com as my email, which i use with mailbox.org, would it be possible to keep using it with mailbox.org and then for mozilla to allow it to be used for email relays? e.g.
name@example.com goes to mailbox.org
3859dhtog@example.com goes to firefox relay
(not currently ofc, but a future thing)