I own a ZTE Open, UK edition. I tried upgrading from 1.0 to 1.2 following these instructions.
The update I installed came from ZTE themselves, and was not a custom, or third-party ROM.
The upgrade worked smoothly and had no errors, and I was prompted to reboot my device. Since then it has been in a reboot loop.
I've tried entering recovery mode and restoring to factory settings, but nothing worked. Likewise, I've tried wiping the cache from recovery mode, but with no success.
Any help would be much appreciated.
EDIT: Also worth adding that I cannot access the phone via ADB or Fastboot.
I had a similar problem when I tried to update to 1.2 using the same tutorial (I have the US model).
The problem seems to be jumping from 1.0 to 1.2. You need to load 1.1 first, then 1.2.
The solution for me was to download both of the ZTE Official 1.1 updates linked on the ZTE Open device page for my region (on the mozilla developer website under the heading "Upgrading Firefox OS" linked below).
Do not change the downloaded files names, just unzip them both and put both .zip files on the root of an sd card and reboot into recovery.
You should be able to flash one of the two files using the install update choice, the other will give an error (I can't read the full filenames on the phone so I'm not sure which one actually worked for me, that is why I say just try both.)
After flashing you should clear cache, wipe data and reboot.
If all goes well you should have a fresh factory load of 1.1 on you phone and you can now follow that tutorial to upgrade to the developer preview of 1.2.
If your recovery won't flash either of the official EU 1.1 updates you may have some how replaced your stock EU firmware with a stock US firmware, one update will say signature verification error and the other one will give another longer error 7 something or other. I replaced my US firmware with an EU firmware (v1.0) without realizing it and thought my phone was totally bricked because I couldn't get any US 1.1 firmware to flash and I couldn't get to fastboot or adb to manually flash a build (I loaded EU 1.0 version then used fastboot to flash a 1.4 build that wouldn't boot). The solution to that problem was to load an official EU 1.1 .zip from recovery and then load the US 1.2 developer preview from fastboot which replaces the EU recovery and let me flash an official US 1.1 .zip build. Try not to mix and match firmwares from different regions, the Open doesn't seem to like that.
I was able to get the ZTE preview of 1.2 to work but it has a nasty bug with the notification bar so I went back to the official 1.1 build so you may want to stay at 1.1. If you do load 1.2, the key to using the notification bar is to tap and hold above the screen and slowly "roll" your finger onto the bar until you can pull it down (that might not make much sense now but if you get 1.2 loaded you will understand).
Hope that helps.
https://developer.mozilla.org/en-US/Firefox_OS/Developer_phone_guide/ZTE_OPEN