Is it safe to use a foam seal to run a generator cord through a garage door?

No, it’s not safe. Carbon monoxide is odorless and will seep through any gap, even a compressed foam seal, putting you and your family at risk. The garage door can also pinch or wear through the extension cord insulation, creating a fire or shock hazard. The only safe way is a professionally installed through-wall pass-through, or keep the generator entirely outside and well away from the structure.