note: I don't work for Mozilla but follow B2G closely. All of this stuff is possible. Everything in B2G is a web app, including the keyboard IME (and the browser, and the window manager itself). You can definitely swap that out. The only thing that's not swappable, to my knowledge, is the phone app.
It's super easy to swap out any piece (or replace the whole UI) if you're willing to mess around on the device, but there's no way to do this from the UI as it stands, like you can with Android. I'm excited to see the solution to that problem.
