I'm sure I'll figue it out eventually, but I don't see this as a priority right now. Obviously, the mention capability itself still works for you in FF - you just have to type the names yourself (which I know, sucks!). In the meantime, I know what's happening and where - just not "why"...
Definitely no rush on this.
I've also tried to manually do it but that didn't seem to work either. I'll test it again now. EDIT: Sweet. It worked.
Yeah, manually writing it out works - unfortunately, you are forced to know the EXACT spelling of the username without that lookup...
...for what it's worth - the drop-down still works in Chrome, so this is a FireFox issue at the moment.