Firefox OS (nightly images) from FreeXperia Team is now available for download for Xperia Z3, Xperia Z3 Compact, Xperia Z2, and Xperia Z1, etc. Firefox OS is still in developing stage so do not test it on your main device.
As seen in video, user mentions that radio and camera functionality is not working on Xperia Z3 running Firefox OS, so it has some bugs which might not be suitable for daily use.
[ VIDEO ] Xperia Z3 running Firefox OS
Check out the video below showing Xperia Z3 running Firefox OS.
If you are looking to install Firefox on your device, download it from here.