Windows 12 2012 R2 is windows version 6.3 that is the same version reported by Windows 8.1 both use build 9600 of the windows code base. So is it compatible, yes as much as Thunderbird is compatible with the windows 9600 build. The only difference in a desktop and server version of Windows is a registry key and some bundled application software. Just as the difference between home and professional versions is also artificial based of registry keys. The underlying core is the same. If you are asking if Thunderbird is tested in a loaded server environment. No it is not, it is desktop user software and is only ever tested on the software a desktop client would use. So you may see issues with server components, but I never did when I used the 2008 version with Thunderbird. BE careful reading those requirements though. They are essentially made up. The disk requirements are at least double what is shown I needed more than the 200mb of disk space to actually install the 64bit version and I would consider 4Gb ram an absolute minimum in a real life usage.