GamePlank

How much of the Master Chief Collection is cross-platform?

I'm aware that MCC offers crossplay between PC (Store/Steam) and Xbox, at least partially. However, I'm trying to find out the exact scope of this support. Are all multiplayer modes cross-platform, or only specific ones? What about Firefight? And is there any plan from 343 to expand cross-platform functionality to other campaigns beyond Halo 3 and ODST?