Add a hook to allow subclasses to specify a firefox profile.

On a couple occasions I've wanted to pass a different profile when starting the Firefox browser. I believe this hook can help with that.

If you think it's a good idea, let me know and I'll make sure to include the docs and whatever else is needed in the request.
This could be simplified to just "self.sel = Firefox(self.get_firefox_profile())"; None is the default value for firefox_profile in the constructor anyway.