Firefox has a serious performance issue. The difference with Chrome is really noticeable after only a few minutes of browsing. Less responsive, huge memory comsumption.
Try d3.js for instance, the rendering speed is horribly slow on Firefox.
I won't be switching to Firefox unless its performances get closer to Chrome's.
As a developper I wish I could contribute to make a better Firefox, but I'm pretty sure the technical level is too high for me.