IMHO, performance. Chrome's V8 engine (for running JavaScript) was way ahead of competition when it was released. It took Firefox many years to come close to it.
Disclosure: Happy Firefox user since v1. Sometimes I was jealous of Chrome's speed, sometimes I was weary of Firefox's memory leaks, but never enough to leave the flexibility and customizability the Firefox provided.
EDIT: ... and now Firefox is fast enough, and isn't a memory hog anymore, so I continue to be a very happy Firefox user.