For what it’s worth, this is a long rabbithole on the whole e2ee thing so just in short:
From what I gathered poking around in the past XMPP’s OMEMO seems to have had shakier protocol design often leading to silently lost messages if you use multiple devices (I asked around among devs what it actually did and why, would take a while to explain) while Matrix usually has noisily lost messages so you can try to resend. So since that happened to me I’ve abandoned XMPP personally, I wouldn’t know why these differences existed or whether it was fixed since then.
(Regarding group chats, I also think e2ee is still useful there, can limit leaks a little if you use history since joining and ban leakers if possible.)
oof, that’s just sad. i hope people switch, firefox is actually not that bad these days