If we undertake it is your average glass pickle-jar with a metal top than heating up the jar will release the lid since the metal demeanors more heat than the glass. This means the metal will enlarge before the glass gets the chance to increase the same distance, so as the metal enlarges the Force necessary to open the jar decreases. It is the force of friction that is equal to the coefficient of friction x the normal force amid the objects. As the lid come loose the normal force the glass makes to push back on the lid is reduced so the Frictional force is dropped.