Sorry at this moment you can not download Firefox for your windows phone. Actually there were some technical and legal issues to put Firefox on windows phone. And currently the engineers of Mozilla is busy with Firefox for iOS and it's own mobile platform Firefox OS. So may be Firefox will not available for windows mobile in near future!

The only mobile OS Mozilla supports is Android. The iOS option is more of a shell.

Even though the early mobile experimentation Minimo was done on then mobile Windows devices, Mozilla has had no plans to make mobile Firefox work on any Windows phone since.