sɣuṛ Joaquín Serna (Aneflay) di Dec. 27, 2021 · aseɣwen yezgan

I've just tested it in version 91.4.1 with a folder with a few emails and it exported. Keep in mind that this extension 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 improve it, I will accept the changes gratefully.