When trying to start with a default configuration using Selenium 4.0 or 4.1 (3.X is now unsupported), the presence of the empty marionette variable leads to an invalid capability exception in Selenium:

The following debugging information is available:
. Could not open connection: Capability value must be set

A Boolean attribute must be a boolean (True/False) and must not be Null.

The marionette value should either set a Boolean default, or not set any at all.

The marionette value is purely present to support legacy versions of Firefox and should be removed.