Selenium Marionette driver UnreachableBrowserException upon second launch

After digging a little deeper, I came to the following findings which eventually solved my issue:

1. Marionette resp. wires uses two ports (see wires --help); a marionette-port and a webdriver-port:

   Usage:    ./wires [OPTIONS]WebDriver to marionette proxy.optional arguments:  -h,--help             show this help message and exit  -b,--binary BINARY    Path to the Firefox binary  --webdriver-host WEBDRIVER_HOST                        Host to run webdriver server on  --webdriver-port WEBDRIVER_PORT                        Port to run webdriver on  --marionette-port MARIONETTE_PORT                        Port to run marionette on  --connect-existing    Connect to an existing firefox process

   When running multiple MarionetteDrivers simultaneously, both ports have to be different from the already running instance obviously. However, when using the default constructor new MarionetteDriver() the marionette-port remains constant (and is not determined based on some free port). We used some workaround (see below) for the GeckoDriverService.Builder to always pick two randomly-chosen available ports.

2. The current (version 2.48.2) GeckoDriverService has an empty implementation of waitUntilAvailable() (which should check, if the WebDriver is ready to go). Sporadically, this lead to the UnreachableBrowserException posted above.

To circumvent these issues, we did something like this at the end:

// determine free ports for Marionette and WebDriverfinal int marionettePort = PortProber.findFreePort();final int webDriverPort = PortProber.findFreePort();// override, as GeckoDriverService provides no direct way to set the Marionette portGeckoDriverService.Builder builder = new GeckoDriverService.Builder() {    @Override    protected ImmutableList<String> createArgs() {        Builder<String> argsBuilder = ImmutableList.builder();        argsBuilder.addAll(super.createArgs());        argsBuilder.add(String.format("--marionette-port=%d", marionettePort));        return argsBuilder.build();    }};builder.usingPort(webDriverPort);builder.usingDriverExecutable(pathToDriver);GeckoDriverService driverService = builder.build();try {    driverService.start();} catch (IOException e) {    throw new IllegalStateException("Could not start the GeckoDriverService", e);}try {    // keep checking the WebDriver port via Socket until it's available;    // as far as I could tell, there is nothing more "high level", e.g. REST API    waitUntilReady(webDriverPort, TimeUnit.SECONDS.toMillis(30));} catch (InterruptedException e) {    // ignore}return new MarionetteDriver(driverService, capabilities);

You need to handle the binary required by Marionette. You can do it by hand (download the binary on your own and export the proper variable), or you can do it automatically using WebDriverManager. Simply add the following dependency:

<dependency>   <groupId>io.github.bonigarcia</groupId>   <artifactId>webdrivermanager</artifactId>    <version>1.6.0</version></dependency>

And then, in your code call to:

FirefoxDriverManager.getInstance().setup();

Mostly UnreachableBrowserException happens when Selenium couldn't find the required exe of the driver. Since you haven't put driver capabilities setup, I am assuming the following steps will solve your issue.

As per Mozilla MDN link Marionette is set as a DesiredCapability

DesiredCapabilities capabilities = DesiredCapabilities.firefox();// Set Marionette on so the Grid will use this instead of normal FirefoxDrivercapabilities.setCapability("marionette", true);WebDriver driver = new RemoteWebDriver(capabilities); 

Also, Marionette executable is added to the path (In Windows):

Adding the executable to the PATH

Selenium will try use the executable in the path. You will need to add it to the path using the following.

Finally, another SO question where UnreachableBrowserException issue is handled.