I have an Open C and I recently manually updated the phone from 1.3 to 2.1. You can most definitely compile and install a new version of Firefox OS yourself regardless of what ZTE chooses to do. The only catch is that you need root access, and this is not enabled on the Open C by default, but ZTE provides a tool for rooting your phone with a single button press.
My understanding is that all aspects of the OS under the control of Mozilla are completely open, but manufacturers like ZTE include proprietary components and of course their hardware isn't 100% open, but Mozilla isn't responsible for that. Mozilla certainly needs to find other partners however, because my experiences with ZTE have been pretty poor thus far, and I've read a lot of negative things about Alcatel's Firefox OS devices as well.
As an aside, Mozilla needs to start targeting higher end devices since I believe that hardware experience will largely factor into the success of the OS, and the current official Firefox OS devices are pretty awful. I've also been following bugs concerning core features for YEARS now, and Mozilla has yet to address any of these. 2.1 has some improvements over 1.3 for sure, but it's missing very basic functionality like some group messaging features and a calendar application that's actually useful. A lot of the noticeable changes from 1.3 to 2.1 are purely cosmetic, and it's incredibly frustrating to see essential features get completely overlooked time and time again. The marketplace is also starting to seem completely devoid of useful applications - a lot of them are very low quality, some of them are simply shortcuts to the mobile versions of websites, and games seem to make up the majority of the applications.