Just like on Android prior to the release of the Native Developer Kit, the company blames it on the inability to develop native applications:
While we think Windows Phone 7 looks interesting and has the potential to do well in the market, Microsoft has unfortunately decided to close off development to native applications. Because of this, we wonâ€™t be able to provide Firefox for Windows Phone 7 at this time. Given that Microsoft is staking their future in mobile on Windows Mobile 7 (not 6.5) and because we donâ€™t know if or when Microsoft will release a native development kit, we are putting our Windows Mobile development on hold.
Mozilla will soon disable the builds and test automation, and will only return to further development if and when Microsoft opens up the Windows Phone platform. The move comes as another blow to Windows Mobile, which is rapidly entering dead man walking status.
Thanks Cristian Adam for the tip.