This generally means that targeting the lowest end of the market isn't a great idea unless you are trying to make money off the sale of the device alone and not ongoing services like apps.

Even if it didn't have any apps, it provides complete and open access to the Internet for $25 to people who might not otherwise have access to it. Wikipedia, email services, OpenStreetMap, Twitter, news sites, political forums, MOOCs. It's a chance to give the last billion people their voice on the open Internet.

Next billion, not last billion. http://www.internetworldstats.com/stats.htm says that of the 7 billion people in the world, just shy of 2.5 billion have internet access. http://www.internetlivestats.com/internet-users/ claims 3 billion. http://en.wikipedia.org/wiki/Global_Internet_usage claims about 3 billion.

All of which is to say that less than half the world actually has anything like consistent internet access today. And yes, getting those next 4 billion people on the internet is one of the explicit goals here.

If there's no way to do something similar on Firefox OS, then I see that as a much worse hindrance to the ecosystem than the income demographic they are targeting.

He's already submitted a git browser: https://marketplace.firefox.com/app/git-browser

The carriers include: Telefonica, DT, Telenor and more...

This is not a fly-by-night project, it has already launched in 14 countries.