Down to Three.
But now, Presto is going to be no more. Opera has decided to switch to Webkit. Well, although probably a smart business move, for people such as myself, who value platform openness, this is a huge negative. Why though?
You might be thinking, "Webkit is open source, the Opera team is now just part of Webkit, so what's the problem". Well, it's really a sense of control that is the issue. Webkit is now dominating the web, especially in mobile. So now, when developer's are testing something, they can't really compare other web engines. It's just, "Wow, that was weird" instead of "Odd, let's just try another engine and see if this is a bug". Webkit's bugs will become the standard. Webkit now controls over 50% of web browsers, which makes our job all the harder.
Firefox needs to deliver open standards, and work harder than before to compete (Though it really isn't a competition) with Webkit. Devs may start not wanting to code on "other" platforms, they might choose only Webkit. Now it's purely up to Webkit to deliver new standards, and to do everything right. Not something we can be sure of when half the team if composed of people from Google, Apple, and Opera.
Although this isn't as bad as it could be (platforms could be controlled by a closed source engine), it is still a huge negative. But this just means we need to work even harder. But there is no doubt we are becoming outnumbered, it's the Mozilla Foundation plus it's developers against Apple, Google, Opera, and the entire Webkit Team. Finances especially, since Apple and Google can pay the big bucks to keep it going, is a big problem.
If you would like to support financially, donate here: https://sendto.mozilla.org/page/contribute/openwebfund
Or, get involved by actually volunteering: http://www.mozilla.org/en-US/contribute/