We really don't use the Android graphics stack. We use the drivers in the kernel, and we use the gralloc stuff, but that's about it. Everything else is Gecko's own renderer.
(I work on the gfx pipeline for B2G)
Edit: Oh yea, and we use Android's libegl wrapper stuff. Not that it matters, but just in the interest of correctness.
I was primarily referring to the drivers architecture (which is incompatible and useless for X.org and Wayland). I mean in practice, Firefox OS targeted devices won't have X.org/Wayland GPU drivers, only Android ones. It's not important for immediate goals of Mozilla, since squeezing regular Linux graphic drivers from vendors is a horror. But it doesn't help other distros (like Mer/PlasmaActive and etc.), if they decide to make ports to these devices. It may be not Mozilla's primary goal to promote conventional mobile Linux, but in practice these devices won't help that cause (at least for now), so I feel like they won't be enough of "open devices" in that sense.
On the other hand, if X.org/Wayland based devices will appear (like upcoming Mer/Vivaldi tablet), Mozilla can enable B2G to run on them as well, since they'll provide sufficient support for OpenGL ES.
(I work on the gfx pipeline for B2G)
Edit: Oh yea, and we use Android's libegl wrapper stuff. Not that it matters, but just in the interest of correctness.