od Joaquín Serna (Vývojář) v Říj. 3, 2021 · trvalý odkaz

I've just tested it in version 91.1.2 with a folder with a few emails and it exported. Keep in mind that this extensions was only made as a proof of concept for bug 752295 (https://bugzilla.mozilla.org/show_bug.cgi?id=752295)
It can be slow and it can get stuck with large sets of emails. It is open-source so if you feel like it you could try to improve it and I will accept the changes gratefully.