It almost doesn't matter. The primary thing you want to prevent is the easy migration of fire between floors. Having openings between the floor and the wall is like having a chimney in every wall bay and this can cause your house to go up VERY quickly, instead of being limited to one room or just downstairs. It gives you more time to get out and increases the chances of saving the house and it not collapsing quickly.
2x4 blocking is ok. One easy way is this special, heavy duty fireblock stuff that looks sort of like fiberglass insulation (not pink, but a more natural light brownish color) that you can cut with a serrated kitchen knife for very quick and easy installs. Just cut it snuggly and pressure push into place, done. you don't want to be able to see light coming from the floor below. Even pipe and electrical chases and holes in the floor are important to seal. Small holes and gaps around ducts and pipes, etc. can be sealed with some of that foam insulation, maybe the make some more fireproof than others. The key is to prevent any air circulation. If air sneaks through there, then the fire will follow chimney style. The fire resistance qualities of the block are (almost) secondary to blocking any air passages between floors.