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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _get_firefox_start_cmd(self):
 """Return the command to start firefox."""

        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