Community builds for Firefox OS provide you with three different options (Beta, Aurora and Nightly) just as the Mozilla Firefox browser does. Those builds are targeted at the owners of French or European variants of the ZTE Open C. Those builds are made possible since Firefox OS is an Open Source project. You will be able to downgrade back to version 1.3 at any time by applying the “root” procedure corresponding to your phone.
There are at least two versions of the ZTE Open C, depending on where you bought it from:
There are very few differences (a light sensor on the FR version, a slightly different camera, etc.) but they are significant enough that you need to choose the right version of the community builds in order to avoid compatibility issues.
To identify your phone version, you need to go to the Settings>Information menu, and then check the Software version.
If the displayed version is different, you don't have a ZTE Open C (it might be a “ZTE Open”, without the “C” : some of them have a version starting with
OPEN_US_DEV_FFOS). In any case, these community builds won't be installable at all on your device. Sorry.