Because it will collapse otherwise because it no longer can suck on Google's teet.
But now, they've got Bing's teat to suck on.
I imagine that eventually people will stop giving Mozilla boatloads of free money and Mozilla will have to figure out a way to function like a real business. However, this isn't it:
"they've hinted at plans to start supporting Android apps"
That will kill Firefox OS faster than anything. If Firefox OS runs Android apps then there's no reason for write any Firefox OS apps. There already are a gazillion Android apps out there. And if you think of something new, and you're already experienced at writing for Android, there's no need to learn how to write for Firefox OS, just keep making Android apps. And if there are no Firefox OS-specific apps then there's no reason to use Firefox OS.
Google The Failure of OS/2