First thing you need to be able to do is to create an XPCOM component. A simple example is in the samples directory of xpcom, mozilla/xpcom/sample.
You'd create a component, then add methods that match your DLL functions. These methods would then make the actual DLL calls. Now that you have an XPCOM component JS can then invoke the methods.
http://www.mozilla.org/catalog/architecture/xpcom/ is probably a good place to visit as well.
Thanks a lot. Daniel
