Please check the following related discussions: https://www.researchgate.net/post/What_is_relationship_between_the_glass_forming_ability_GFA_and_the_viscosity_of_the_corresponding_liquid_phase https://www.researchgate.net/post/Does_the_glass_forming_ability_GFA_depend_strongly_on_the_composition_of_the_glass_system https://www.researchgate.net/post/Thermal_stability_of_glasses
Glass forming ability considers the ability to form a glass on cooling. Thermal stability considers the tendency to crystallize of a sample which has already formed a glass during reheating or holding in the crystallization temperature range. In other words, a sample must first be a glass (cooling) before it can be considered for thermal stability against crystallization (heating).