Seeds don't have to "breathe." I save seeds in plastic, BUT be very sure they are completely dry before storing them, or you will end up with a plastic bag of moldy seeds. With larger seeds (like tomato), you can test for dryness by trying to break the seed in half -- if it cracks into 2 pieces, it's dry, but if it bends it needs more time. (And obviously that particular seed will not germinate.) With smaller seeds, I just leave them out on a paper plate (be creative if you've got pets or kids; protect your seeds from being accidentally scattered) for several weeks, just to be sure.
Store them in a dark, cool place... controlled room temperature is fine; you just don't want to tuck them away in a hot attic. Mine are in a chest of drawers in the living room! :-)