I'm sure some won't like this comment, but there are only two that matter: Chrome and Safari. Firefox's share is about the same as IE 11 (2.0 -> 2.5%).
EDIT: This is on laptop/desktop. Without the device filter it's about 5%. But then again, I don't think it comes by default on any Mobile OS, so they have a disadvantage there. Plus non power users of their smartphone (like me) might not change the browser there.
That's may be true, but that doesn't mean Firefox/Gecko isn't a viable alternative just because fewer people are choosing to use it. In fact, I think it demonstrates my point that people don't need competing browser engines that badly because they aren't radically different. There was a time where IE was hemorrhaging users to Firefox, but that was because Firefox legitimately did things better than IE/Trident. At this point, however, our expectations of what a web browser should do are fairly solidified, so people are likely going to choose whatever is being marketed to them the most and comes installed on many Android phones by default.
