Discord has aggressive security measures. I can't use it, because it won't let me access it from my desktop unless I give it a phone number. I guess it's decided I look like a headless browser or something.

Certainly I'd say discord is a poor fit for an organization allegedly dedicated to web freedoms, such as mozilla.

I'd suggest rocketchat or mattermost, personally. Well, to be honest I'd suggest IRC, but if that must be abandoned, rocketchat is pretty good.