createAdditionalPantherClient() with firefox driver not working.

[yedprior]

Opening secondary client when createPantherClient use firefox driver send
"Facebook\WebDriver\Exception\SessionNotCreatedException: Session is already started" exception when
$client2->request('GET', '/chat');
is called in this example :

        $client1 = self::createPantherClient(['browser' => static::FIREFOX]);
        $client1->request('GET', '/chat'); 
 
        // Connect a 2nd user using an isolated browser and say hi!
        $client2 = self::createAdditionalPantherClient();
        $client2->request('GET', '/chat');
        $client2->submitForm('Post message', ['message' => 'Hi folks 👋😻']);

        // Wait for the message to be received by the first client
        $client1->waitFor('.message');

        // Symfony Assertions are always executed in the **primary** browser
        $this->assertSelectorTextContains('.message', 'Hi folks 👋😻');

It works fine with chrome driver (open 2 windows with navigator).

[halfer]

I suspect this is by design:

mozilla/geckodriver#1946