Recently I did some research on using Firefox as browser to debug asp.net webapps. Andre points out in a comment this does not seem to work with webservices. There is one thing you have to watch to debug these.

When you start a webservice the browser pops up and displays an overview of the service. Clicking a method will fire up a new browser window. Using FireFox you will see a new instance of ff popping up. This new instance is a new process, not the same process the VS debugger had attached itself to and you will miss the breakpoints. But when you invoke the webmethod in a new tab in the same instance of Firefox (right mouse button) the invocation will be in the same process and your VS breakpoints will be hit.

Peter