Looking further in webbrowser.py, FF uses GenericBrowser:

class GenericBrowser(BaseBrowser):
    """Class for all browsers started with a command
       and without remote functionality."""

   ...

    def open(self, url, new=0, autoraise=True):
        cmdline = [self.name] + [arg.replace("%s", url)
                                 for arg in self.args]
        try:
            if sys.platform[:3] == 'win':
                p = subprocess.Popen(cmdline)
            else:
                p = subprocess.Popen(cmdline, close_fds=True)
            return not p.wait()

If FF is already open, Popen.wait() returns TRUE for FF but FALSE for IE. This leads to fallthrough for FF and then calling open for IE.