The ZTE Open is actually just a cheap Android device. Firefox OS runs on top of Android. If you don't mind getting your hands a bit dirty, the current build scripts appear to support a number of old nexus devices (galaxy-nexus, nexus-4, nexus-s, nexus-s-4g, and the galaxy-s2 among others). I have not personally installed a build I've compiled on anything other than a ZTE Open, but from what I read it's too difficult - maybe a weekend's worth of work.
Firefox OS does not run on top of Android. The Android Linux kernel is used but that is it. All the Java/Dalvik code was removed well before the initial release.
I may be splitting hairs here, but I'm fairly sure it's a bit more than just the kernel. Fastboot, and ADB function identically. The partition/imaging layout is identical, so much so that I was easily able to run CWM Recovery on my ZTE Open. On top of that the build processes are very similiar and a large number of repos that appear to be forks of AOSP are included. Perhaps there's another name for this shared part, but I just figured all a good part of the native base was borrowed from Android.
If I made it sound like Firefox OS was just a fancy application launcher, that was not my intent.
