Foam will dull any kind of blade, scissors or sharp edge very quickly, but if you put just a tiny bit of oil (3-in-1 oil is best in my experience, but even vegetable oil or olive oil will work if you don't have anything else) it will extend the life of the blades and save you a lot of money.
I usually put a drop or two of oil on a scrap piece of foam and then run the cutting edge of the blade through the oil once or twice, you should be able to see a thin coat of the oil glistening on the blade. You don't need or want a lot, just enough to lubricate the surface the sharpened edge that actually cuts the foam. Put the blade through the oil drop(s) to re-lubricate it every 2-4 cuts.