Editable from field is also the only reason I was using it with protonmail. I can't remember if it was a protonmail limitation or a Thunderbird one that meant I had to pre-add any email addresses before I could send any messages from them though.
Now I'm on fastmail, the from address is an input box that I can put anything into.
Now I'm on fastmail, the from address is an input box that I can put anything into.