Mozilla/5.0 (Windows NT 6.1; WOW64; rv:15.0) Gecko/20120824 Thunderbird/15.0
Maybe "uname" could be an option to detect mingw? On my system it
returns "windows32" for non-login, and "MINGW32_NT-6.1" for --login
shell. Cygwin returns CYGWIN_NT-6.1-WOW64, so it won't be mistaken as mingw.