Ferroelectrets with good piezoelectric coefficients have been produced based on cellular polyethylene (PE) via continuous extrusion film-blowing process. The latter was used to provide substantial biaxial stretching, which is important to decrease the elastic stiffness in the thickness direction and to provide more surface for charge capturing via cell deformation. Thermal and chemical treatments were then applied on the developed films to increase time and thermal stabilities of their piezoelectric activity. The quasi-static piezoelectric coefficient (d33) value obtained (935 pC/N) was well above typical values for polypropylene (PP) considered as the workhorse of piezoelectric polymers. Firstly, films crystallinity was increased via thermal annealing at 80 °C for 5 minutes leading to a 32% increase of the initial d33 value. However, this thermal treatment did not improve the thermal piezoelectric stability of the films since they showed a significant decrease of their piezoelectric coefficient d33 (around 96%) at 80 °C. Therefore, these films were treated with orthophosphoric acid resulting in substantial charge stability improvements, especially at higher temperature. Overall, it was possible to increase the continuous service temperature (CST) of PE ferroelectrets from 40 to 80°C, which is similar to the typical CST of PP.