Nightingale is supported on the main 3 platforms : Windows, Linux, Mac OS X.
Contributors are mainly on Windows and Linux.
We only build x86 builds for Windows
Nothing special, XP is supported by Microsoft until 2014
Since most of users are on Seven or XP we are not testing it and Microsoft is mostly deprecating this.
Ubuntu is one of the main public distro, one of our contributor is working on Unity support, and it should be proposed in the first run add-on bundle when it's done/tested.
Ubuntu is shipping already a media player : Rhythmbox with Ubuntu One and Music Catalog (7Digital)
Thanks to Apple, Mac OS X is the most difficult platform to support, because of the hardware price, platform being locked more and more and iTunes being use by most of Mac users.
Currently we are building using VM or physical machines (Mac Mini)
It seems like native borders doesn't work good
Some add-ons were made to support multimedia keys on Apple Keyboards.
Retina support is on latest Firefox releases, as always we should update our core to support this.
We doesn't support mobile platforms yet, but we may work on a Web version.
There already a lot of good music players on Android :
OS in development, interesting because it is only made using web technologies.
A music player is already in development but I doubt it will easily allow add-ons.