Mozilla has been trying very hard to get its Firefox OS off to a good start. After announcing two Firefox phones at MWC 2013, it has now chose Sony Xperia E to run an experimental ROM of Mozilla's latest OS. It has chosen Xperia E to be the mule and not a more powerful smartphone because Firefox OS will be targeting lower end of smartphones and also, as it resembles Firefox based two phones - ZTE Open and Alcatel One Touch Fire - in specifications.
As Sony points it out, the ROM isn't yet ready for daily use as it doesn't support wireless connectivity(of any kind) as it hasn't passed certification and also micro SD card slot and touch screen are kind of buggy.
To install the OS on your own Xperia E, you will have to flash it. You can find instructions about it by clicking here.
As we know, Sony already announced that they have plans to launch a Firefox OS powered device in the coming future which is why Sony decided to launch this experimental ROM. It will give chance to app developers to test the waters and give Sony valuable feedback about the OS.