How can I uninstall Firefox OS? I was handed a phone from my friend who installed Firefox OS over Android, but now wants Android back. I know roughly how to flash ROMs, but am not sure how to go into download mode or equivalent in Firefox OS. Any help would be appreciated.

The model of the phone is a ZTE Kis. If I can supply any more information just ask!
Steps to Access Download Mode on ZTE Kis 3 Max


Turn off the device
Remove the memory card from the device
Press and hold Volume Down key + Volume Up key simultaneously
Press and hold Power key until the device completely freezes.


You can flash a ROM as usual after that. The stock ROM for your phone can be downloaded here.

Source