Where has marionette.defaultPrefs.port done in prefs.js in firefox 55? All my scripts stopped working after update :(

I just looked in about:config and it isn't there at all nor any other port related setting I could see.

I need it for setting the marionette port which I was doing programmatically when I would start new instances of my script. I would have them edit that line to choose a random free port and add that to the file so the marionette python driver could communicate with firefox. Now that setting is nowhere to be seen.

This is why I avoid whole system upgrades as long as I can.

Where is that setting hiding now or has it been deprecated or something?

I had been using firefox 51 before upgrading today.

EDIT:

ah this article seems to cover it.