The only way to get back to a good known state is by waiting for that socket to timeout on its own.
== Socket 2828 is unavailable ==
If you're running the suite by hand and you have aborted a previous run, you might have left firefox running.
If you
MarionetteException: MarionetteException: localhost:2828 is unavailable.
You can see the socket held by firefox you can run this command:
$ netstat -anp | grep ':2828 '
# TODO add output of an instance
You can fix it by running `killall -9 firefox`.