MimiBug,
No way in the world could a Class 66 bobbin work in a FW. I just went down and tried one in my FW and the bobbin case would not even fit all the way in. The 66 bobbin is almost twice as thick as a FW bobbin.
So, if you did use a 66 bobbin in your machine, I'm going to have to agree with thepolyparrot, you don't have a FW.
Joe