The very low market share with Windows Phones makes it not worthwhile to dedicate essential resources and money to make a mobile Firefox version for such devices.

Mozilla would have made versions for Windows Mobile starting some years ago but like would have stopped due to low users.

http://www.windowscentral.com/just-1-market-share-why-are-oems-choosing-windows-10-mobile

Initially when Mozilla experimented with a mobile browser they did so with Windows mobile OS's then as it was Minimo. Later Mozilla changed to Android as the paltform for mobile Firefox.

Unless you were asking for the Firefox OS for to be used on phones that came with Windows Mobile however it is a discontinued operating system except for maybe still on some Panasonic Tv's. https://en.wikipedia.org/wiki/Firefox_OS