The problem with FireFox is that it wasn't fast enough to catch with the innovation in the browsers market. Google Chrome was getting fast and pretty good at rendering. Its minimalistic interface has also helped a lot.

By the time FireFox figured that out, it was already a little late. But the problem is that they approached it the wrong way. Instead of focusing on what made Google Chrome successful (simple interface, auto-updates, speed...) they even made it worse (I still find the interface not easy and friendly as chrome, multiple confusing versions, and not that fast)