I don't know exactly how things went down inside Mozilla, but I think their refusal to ditch their old extensions API was the biggest thing holding Firefox back. This wasn't due to poor engineering chops; I believe they simply didn't take the decision to inconvenience millions of users - many of whom were using FF perhaps solely because of the vast extension ecosystem.

I can't hate them for some of the projects they tried. FirefoxOS was actually something the world really fucking needed. We need an open mobile OS.