With the season drawing to a close Thistle seem to be hitting a loss of form at the worst possible time.
Their promotion hopes were dealt a blow last Saturday as they succumbed to their second consecutive defeat.
It took only eight minutes for the Dunfermline side to take the lead when Williamson ghosted in behind Bell to collect a Walker cross before slotting the ball past Henderson.
Rosebank doubled their advantage when Forrester headed home unmarked and then just a minute later it was 3-0 with Forrester again the scorer taking full advantage of some poor defending.
Thistle needed to come out of the blocks quickly for the second half, but instead they gifted Rosebank a fourth goal just three minutes after the restart.
They failed to cut out the run of Gray who slipped the ball through to full back Murphy and he blasted a low shot past Henderson.
After 71 minutes the rout was complete when Ross failed to get any distance on his clearance with the ball falling kindly for Forrester who drove the ball home to grab his hat-trick.
Thistle grabbed a consolation goal in the 88th minute when debutant Archibald held off Close to knock the ball into the path of Jukes who curled the ball round McKinnon.
A sore defeat for the Balmullo side who got what they deserved after serving up a 90 minute horror show.
But with four games remaining they need to regain their mid season form if they want to keep their promotion dreams alive.
Thistle – Henderson, McLaren, Gorgon, Carson (Capt.), Bell (Petrie 46), McShane, McArthur (Jones 67), Moir (Archibald 76), Band (Ireland 59), Ross, Jukes.
Sub - C Murray
Tomorrow (Saturday) Thistle play host to Glenrothes Strollers and then on Tuesday they travel to face Kirkcaldy Rovers.