Google's anti-competitiveness is easily the biggest reason Firefox doesn't have a majority market share. I would argue, however, that several of those things are the reasons Firefox can't hold onto a minority share either. Sure XUL, themes, tabs on bottom, etc were never going to keep Firefox on top but they might have been enough to hold onto 10%.

Google is evil so I still use Firefox, but it just feels like Chrome without Google. The evangelizing passion I had for it ever since version 1.5 is dead and Mozilla killed it.