Earlier this year, I spent some time developing for FirefoxOS, a Mobile OS based on a linux kernel and Mozilla browser. To be honest, I have a Revolution phone from Geeksphone which flashes between Android and FirefoxOS, and it has been Android since day one. It work well for what it needs to do as an Android device. However, the idea of an instant developer community for FirefoxOS is compelling - it just hasn't appeared yet because the OS hasn't become a large enough target yet. And, like ChromeOS before it, this may never happen.
MatchstickTV is a recent kickstarter project that takes FirefoxOS and uses it to run an HDMI dongle very similar to Chromecast, but based on open source apps on an open source development platform and an open source OS. It's a little cheaper as well, but I suspect that is not an important selling feature. On the other hand... because there is nothing in particular to license here, this sort of thing could become a great conference giveaway over time, just like USB sticks used to be.
Honestly, I think a tablet using FirefoxOS with a bunch of onboard educational applications - similar to the original OLPC program, perhaps - would be a really good thing. There is a niche for browser-based mobile, and Mozilla is doing a lot of smart, good things to capture it.