In Short

Firefox OS is Mozilla's open-source operating system primarily aimed at phones and tablets. With the Firefox browser at its heart, Firefox OS aims to use open standards to build a fully open-source mobile OS from the ground up — that's in contrast to Android, which still relies heavily on proprietary code despite technically being an open-source OS. As Firefox OS is a web-centric system, app development for the platform is focused on web-based languages like HTML5 and Javascript.

Mozilla has partnered with some Android phone makers, most notably ZTE, to produce devices running Firefox OS out of the box. So far Firefox OS has been aimed at entry-level handsets in developing markets, and has done little to challenge Android or iOS in other parts of the world. However Firefox OS's open-source nature has meant it's been previewed on a number of unlockable Android devices, such the Nexus 4 and Nexus 5.

Whether Firefox OS will emerge as a true competitor to Android in the developing world remains to be seen, however Mozilla will face increasing pressure as Google and Microsoft look to strengthen their presence in emerging markets.