- Edited
banger Thank you. I have no idea what's going on. I did some spelunking in the depths of Mozilla's bug list and that error seems to crop up from time to time. Anyway, I installed the flatpak and it works. Fine by me.
EDIT: That didn't age well. FlatPak Firefox can't communicate with KeePassXC. I downloaded Firefox 130.0.1 from mozilla.org and installed in ~/ and the mail button works. It seems it's not a Firefox bug after all. I dunno.