code: def _get_firefox_start_cmd(self):
          
          start_cmd = ""
          if platform.system() == "Darwin":
              start_cmd = "/Applications/Firefox.app/Contents/MacOS/firefox-bin"
              # fallback to homebrew installation for mac users
              if not os.path.exists(start_cmd):
                  start_cmd = os.path.expanduser("~") + start_cmd
          elif platform.system() == "Windows":
              start_cmd = (self._find_exe_in_registry() or self._default_windows_location())
          elif platform.system() == 'Java' and os._name == 'nt':
              start_cmd = self._default_windows_location()
          else:
              for ffname in ["firefox", "iceweasel"]:
                  start_cmd = self.which(ffname)
                  if start_cmd is not None:
                      break
              else:
                  # couldn't find firefox on the system path
                  raise RuntimeError(
                      "Could not find firefox in your system PATH." +
                      " Please specify the firefox binary location or install firefox")
          return start_cmd
docs: Return the command to start firefox.