Grain of salt warning: Drew Devault’s (nearly 2 year old) review said Tutanota did not support your own domain, but Tutanota says they do, for paid accounts, now.
Posteo gives 2 GB storage for 1 Euro per month, versus 1 GB from Tutanota for the same price. If you don’t need to use your own domain, I couldn’t find a bad review of Posteo.
go back to Security, enter 2-step verification, scroll to Authenticator
get FreeOTP+ (I tested this) or other and scan the QR code to setup
go back, go to Security (physical) key, remove it (so it does not depend on this Firefox instance, eg. doing this in Private mode will destroy the virtual usb token lol)
save backup codes, put them somewhere safe
Great, now, go back to Security, App passwords, set as many as you want. I tested with Fairemail from F-Droid and Thunderbird and they work fine.
@M66B would ^^^ be a good candidate for FairEmail FAQ update? There’s a lot of strike-through text in your FAQ and the app has 2 buttons for OAuth and whatnot that just say “NOT FOR YOU!!!11”