Hi there- I use bio oil to rub into the stretch marks on my tummy and onto the dark skin under my breats and at the tops of my legs- a lot of the places where I have excess skin have turned dark over the years and bio oil is supposed to help this as well as stretch marks. I don't think you should use it for firming- I, like many others, swear by Dove firming lotion for this, though once I have slapped both on I could slither across the floor like a seal!!
I use it on my face at night.......... don't laugh! I get really inflamed, red cheeks and bio-oil calms the redness down. Lucky I sleep alone though I look like a shiny tomato when I get into bed
Also use it on my c-section scar. As others have said, its not really for firming skin just for calming it down and smoothing it. HTH