I don't think Frontier has switched the seat pan style to lower the weight, except as an incidental benefit; they've done it to make the seats cheaper to build. As a PP noted, Frontier's seats now have NO padding; they are only vinyl covers pulled over metal-framed mesh seat slings. This photo, taken from the side, reveals the actual edge of the sling pan showing through the vinyl cover in the economy seat on the left.
I know upholstery; it isn't just the weight of the foam; it's the weight of the extra fabric that goes on the sides and lower surface, and the fasteners that hook the pieces together -- even thread has weight, and an average padded seat cushion uses about 100 feet of heavy-gauge thread, plus 20 or so staples and some glue. Still, all of that probably only equals out to the weight of about 3 grown men for an entire aircraft, though the fuel savings probably does add up over the life of the seat.
What they are really saving is labor/materials cost, and shipping cost from the mfr. to the airline. It's immensely cheaper and faster to sew up a sling seat cover than it is to create a foam-filled upholstered one. Even with my home machine, I can run up a 2 layer sling in about 15 minutes doing them one at a time; factories use computer-driven saws to cut 50 layers of fabric at a time and machines that do 2K stitches per minute, so I'm guessing that sewing them together takes maybe a minute each, as opposed to the 30 minutes or so that putting together all the components of an upholstered foam seat pan will take. It uses a lot less fabric, no glue, and you can probably ship them 60 to a carton, as opposed to about 24 to a carton for foam cushions.