We investigate the static friction behaviour of model molecular ultrathin films confined between two parallel walls by Monte, Carlo simulations in the recently introduced Grand Isostrain ensemble. The friction curves (i.e. the shear stress as a function of the shear strain) have been computed for two different models (a commensurate and an incommensurate model), and for different temperature, normal compressional stress and film thickness conditions. When normalised with respect to the coordinates of the yield point (determined as the maximum of the friction curve), all the computed data happened to fall on a single 'universal' curve. A new model was derived from this observation. In this so-called elastic model, the behaviour of the quasistatically sheared system is solely dependent on the elastic properties of the unstrained film. The elastic model contains no phenomenological parameter. All the involved quantities may, in principle, be obtained experimentally.