If you have a Flame reference device and wanna try out alternate versions of Firefox OS apart from the stock one, but not willing to build from source, then follow this mini-manual.

Get the build

You can download the packages from the Nightly Build directories of Mozilla FTP. You specifically need the following two files:

• b2g-XX.0a1.en-US.android-arm.tar.gz (XX is the version number)
• gaia.zip

Set up environment

Once you have the build, decompress both of them in the same directory. Download the flash.sh file from this gist and put it into the same directory as well.
N.B: You will have to set executable bit to the script file ($ chmod a+x flash.sh).

Flashing the device

1. Enable remote debugging in Device's Developer Settings
2. Connect the device to the system over USB
3. You will need to have have ADB installed 3.1. Run $ adb devices3.2. Check for Flame in the listed devices 3.3. If device is listed, proceed to step 4 (if not, troubleshoot)
4. Run the script to initiate flashing $ ./flash.sh
5. Follow the instructions to customize your flashing as per your need.
6. If you face issues, try flashing with /data partition formatted when asked.
7. Profit!

Updates

After you're done flashing, your device will be on the Nightly channel, receiving updates almost each day. Those updates will be over the air (OTA) download of ~60MB, and completely hassle free.

Credit: 

Thanks to Deb. :) https://gist.github.com/debloper/e7d194ddb7c1011bbeda