The o-rings should all be the same. Firestone made many different models of corny kegs, some with slightly tricky poppet valves. Do yours have the 3 prongs that wedge into the inside of the shaft? If so, you can try gently prying the prongs loose with a small flathead screwdriver. They can be a pain, and many people either replace them with more standard ones, or just soak the whole post assembly without removing the poppet to clean.