PANTHER_NO_HEADLESS not working with Firefox

[ThomasLandauer]

With this in my .env:

PANTHER_NO_HEADLESS=true

... I'm getting the exception

Process unexpectedly closed with status 1

... on the second (!) line here:

$client = Client::createFirefoxClient();
$client->request('GET', '...');

I haven't tried any other Panther settings in .env, so I can't say if this system working in principle.