Last week the ZTE Open C (a variant of the ZTE Kis III running Firefox OS instead of Android) was sold for just 29 € by ComeBuy. Given such a low price I just had to order one as a “trow-away” phone for concerts and the likes.
Turns out the device is quite good. Overall build quality is better than the Flame’s and its touch panel does not suck.
Sadly the device ships with Firefox OS 1.3, based on Jelly Bean, so Mozilla’s current B2G-builds do not work here. There are community builds, but they still feel quite unpolished, so I had a look for alternatives. Turns out there is a CM 12.1 (Android 5.1) based ROM available and it works great so far.
Update 2015-05-23: Still no problems and Firefox for Android runs fine on the device, too. As it is much more powerful than the Firefox OS browser, using Android is the better choice for me at the moment.