You know, you can build Firefox, Thunderbird, Seamonkey and Xulrunner as Windows x64 binary by my work. Now, Firefox x64 is tier 2. (automation builder builds binary, runs automation test and nightly is available.). I am interested in porting new platforms
Android x86
It already works on gingerbread of Android x86 and will work on Honeybomb. Bug number is 681202. Also, Opera mobile already runs on Android x86
Windows/arm
Required Visual Studio 11. There is a lack of important libraries such as wsock32 to build binaries. I believe that it will be fixed by Visual Studio 11 beta (actually CTP, not beta!). Also, bug number for my status is 686843.