firefox/remote/doc/marionette/Prefs.md

Preferences

There are a couple of Remote Agent preferences associated with the Gecko remote protocol. Those listed below are additional ones uniquely used for Marionette.

marionette.debugging.clicktostart

Delay server startup until a modal dialogue has been clicked to allow time for user to set breakpoints in the Browser Toolbox.

marionette.port

Defines the port on which the Marionette server will listen. Defaults to port 2828.

This can be set to 0 to have the system atomically allocate a free port, which can be useful when running multiple Marionette servers on the same system. The effective port is written to the user preference file when the server has started and is also logged to stdout.