Supposedly FirefoxOS has a number things going for it:
- FirefoxOS needs less resources to run than an Android phone
- there are 6 million webdevelopers and 'only' a couple of 100000 app developers for Android and iOS combined, many are even HTML5-apps with a native wrapper
- it's open source and I've heared it has an update process which updates the top part of FirefoxOS (so not the kernel and binaries) on regular basis. Just like Firefox and Chrome on the desktop
- there are still a lot more people with feature phones than smartphones. There are 6 billion phones in use, only 1.1 are smartphones. Most of these people don't life in a western country and don't have a smartphone partly because of price. A FirefoxOS can already be had for only 3 euros a month (for the fist 3 months, data is free the first 3 months).