After 5 months, most of the swelling and firmness associated with breast augmentation (either above OR below the muscle) should be resolved, but it can take up to a year for full sensation return, scar maturation and softening (internal and external), and "final" implant position and softness. But at this point in your recovery, only subtle or minimal additional changes can be anticipated.
So how firm is "firm?" This can depend on the type of implant (saline vs. silicone), cohesive vs. investigational solid silicone "gummy bear" implants, the size of the surgical pocket, your own degree of muscularity and the thickness of your pectoralis muscles, and whether or not you have begun to develop capsular contracture.
These are impossible to answer without more information and a physical examination. This is best done by your own plastic surgeon, who can answer your questions better than any of us here on-line. Make an appointment to see your surgeon! Best wishes!