If it's any consolation, you can build and install FirefoxOS on a Nexus 5. Note, we are still working on Lollipop support.
https://developer.mozilla.org/en-US/Firefox_OS/Preparing_for...
https://developer.mozilla.org/en-US/Firefox_OS/Preparing_for...
http://bluishcoder.co.nz/2014/06/11/dual-booting-android-and...
The main issue is lack of support for some Nexus 5 hardware, like the camera. Bugs raised for this are not dealt with as the Nexus 5 is not a supported platform.