> Does it support third party browsers yet?

Firefox OS is a browser. Asking if it supports other browsers is like asking if the Linux kernel supports BSD, Windows and Darwin.

In other words, if you replace Gecko in Firefox OS, you are left with nothing of Firefox OS. Just the underlying Linux kernel basically. So technically this just makes no sense.

The situation is different on iOS, Android, etc. - web browsers are just another app there. Many browsers run on Android, and the only reason they do not run on iOS is policy, not technical matters.