It worked in FF back when mentions were first implemented. The last major version release was 2 weeks ago. I can't tell if it was functioning prior to that or not.
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"...