I made that program a long time ago and I will need to go double check it to see if there is something not working, but the program works like this (Same way task manger works it)
The program sends a SendMessageTimeout to a process, and if it gets a response then the program isnt hung up or frozen since it responded. But if a program doesnt respond then (Which is when you see program not responding in task manager) then the program counts that as a possibly frozen process.
But if firefox.exe is responding to that command then it may not be fully frozen and why the program isnt seeing it and killing it.
Or perhaps Windows 8 isnt giving permission for the program to kill it, it should be running as administrator, but just in case make sure you are
Shane