Firefox is developing its own Mobile OS too and latest news is coming from Ubuntu team, Ubuntu is one of the popular Linux platform on desktop and best part is that its free. Ubuntu team is currently developing new mobile OS which will be released very soon.
Currently there is a developer version released of Ubuntu Touch OS which you can install and test on your Nexus Device.
If you are having any of below device then you can test this brand new OS :-
Now connect your phone to your computer and copy both files which we had downloaded above and reboot your computer into recovery boot by holding down the power button, volume up and volume down buttons at the same time.
Use volume buttons to select recovery mode and power button to enter recovery button
Now select Install from Zip option and install both files which we had downloaded and copied.
Now once you had install both files reboot your device and it should boot in Ubuntu Touch OS.
P.S. As its developer preview so many features might not work and crash.