the official position on this is that FF will not work as a plugin in a 64 bit system. however, at least on a vista 64 bit system, FF installed in the program files (x86) folder, which is the compatibility mode on windows, i think, and it then ran just fine as a 32 bit application, including as a plugin. i dont have win7 yet, but it shldnt be that different. did you try installing it on the c:\ drive or did you put it on a different drive? it might only work getting it into the program files (x86) folder if you allow it to install on c:\. just a thought.
If wishes were horses... there'd be a whole lot of horse crap to clean up!
Craig