How do I use selenium to connect to an existing FireFox instance through debugger port?

I would like to startup Firefox and geckodriver seperately, but it seems that selenium didn't export an interface for running a geckodriver which preprares for connect to an exisiting FireFox instance.

For chromium, I can use the following code to finish this task:

chromium = subprocess.Popen(
            [self.chromium_path, "--headless", "--no-sandbox",
             "--remote-debugging-port=" + port])
ops = selenium.webdriver.ChromeOptions()
        ops.debugger_address = "127.0.0.1:" + port
browser = selenium.webdriver.Chrome(self.chrome_driver_path, options=ops)

How do I run FireFox in this way?