While I predict that this comment thread will consist almost entirely of people still complaining about that one time Firefox changed the UI back in version 4, it's nice to hear an actual plan coming out of Mozzy for the first time in a while. Go faster, go multiprocess, make the big architectural changes a program needs from time to time to keep up. So long as Gecko still remains as consistent and lovely as it currently is, I'll be happy with speed and resource upgrades.
A faster UI is a nice idea, but ditching their markup languages is going to screw over a lot of current extensions, and at a time when they're also changing the LAF of the program. I can see that going down like a ton of bricks with people obsessed over which order their toolbars are in.