I think you'd have to copy Netscp.inc, or whatever it is called (several different ones are shipped) and make mods for it to work. This doesn't seem easy, but if you only need to get a few things working it might not be that hard.
Has anyone tried the Mozilla.org site? This might help with your class Id(eventhough i've never used SilkTest or tested using the Firefox browser). However it might be hard to get Firefox to work with your software being that this browser is relatively new on the scene. Take some consolation that Mozilla are the same group who created the present Netscape browser.