For a long time we have written the port Marionette listens on to the marionette.port preference in the profile. We may not be flushing the user pref file correctly, but that’s fixable. We should change geckodriver to ask Firefox to bind to port 0 in order to have it atomically allocate a port, rather than to have geckodriver poll for free ports. This can lead to problems described in https://bugzilla.mozilla.org/show_bug.cgi?id=1240830 and https://github.com/mozilla/geckodriver/issues/1452.

Isn't that a dupe of bug 1421766?

You’re right. I’ve updated the description to be a bit clearer.