I guess it is not based on any general distribution at all but rather compiled in something akin to "buildroot" or similar.
You'll probably miss everything that would do UI/GFX stuff - except if that happens to be Qt. But that won't certainly be the case for Firefox/Mozilla - they've always been directed towards GTK (though Fennec will certainly be different). UI/GFX will probably be the hard part.
I'm pretty much interested in this, though. Not much spare time, though, but interested enough to give it a look on the weekend. So I'm interested in what you find and how far you get!
PS: Memory might become a problem, though:
https://wiki.mozilla.org/Mobile/Metrics/Tracking