Don't call it Styrofoam
Here’s a scattering of references from articles in The Baltimore Sun in recent months:
Styrofoam packing peanuts
tie a Styrofoam cup to a long string
ornaments made of Styrofoam and peas
nary a Styrofoam clamshell or piece of plastic wrap in sight
200 tons of debris in the harbor each year …much of it Styrofoam and light plastic.
eliminating Styrofoam containers
replacing Styrofoam coffee cups with "ecotainers"
The problem: It is unlikely that any of those references are actually to Styrofoam.
The Dow Chemical Co. manufactures a product called Styrofoam, an expanded polystyrene foam used for thermal insulation. It is a blue material that you can see being applied to the sides of houses under construction.
Disposable drinking cups and food containers, packing peanuts, coolers and similar products are not made of Styrofoam, but of an extruded polystyrene foam, typically white, made up of tiny beads.
Colloquial usage may have made styrofoam the common term for plastic foam cups, but anyone who aspires to be an exact writer will avoid it.
This is not an exact parallel with cases of other trade names that turned into generics. A xerox or a kleenex may not be a Xerox or a Kleenex, but in both cases the name brand and the generic product are the same kind of thing. Styrofoam and the plastic foam used in drinking cups are different substances, and we generally find it convenient to have different names for different things.