The market share of Windows Phones is very tiny compared to Android and iOS devices. The share went from 3% in 2014 to 1.7% in end of 2015. So even if they could, does it really make sense to spend resources to make a version of mobile Firefox for a tiny market that is getting smaller over time now?. Mozilla originally started on the then Windows mobile OS's with the [http://www-archive.mozilla.org/projects/minimo/ Minimo] experiment before switching to Android for the mobile Firefox. If Mozilla was going to make a mobile Firefox for Windows phones they would have done so for years, though I would not have been surprised if they discontinued development in last while if they did.