We have a summary of the preliminary report in our briefing files (we operate CRJ 200's).
It states that the left engine thrust lever became stuck at t/o thrust lever position and pilots could not move lever. Right engine controls OK.
After return and taxiing to gate, the right engine was shut down with the thrust lever, but the left engine remained operating at close to full power. After parking and just before disembarking of passengers started, the a/c started moving and accelerated to finally end up in the lounge.
There is no indication in the report of the pilots attempting to shut the left motor down using the Fire switch and considering the fact that the motor was "still running at high power" in the bay, I guess that no attempt was made to shut the left motor down with the fire switch. I assume that the prelim report would have mentioned a failure of the electrical firewall switches.
It does state that they are investigating interference by the cowl strut with the thrust lever mechanism and also investigating the crew handling of the initial failure.