Nope, never heard of Firestone sparkplug giving 40% better gas mileage (mile per mile, if swapped with an OE spark plug of similar age and condition).
Although I too, listen to George Noury radio but I find some of his stuff are too far-fetched to be true.
Most infomercials feed off of those meek-minded individuals like cars that haven't received tuneup for ages then throwing anything new into it will definitely gives/restores good fuel economy and much more. Try it on a well-maintained engine with good quality spark plugs (NGK for example) and I bet you will not see much differences except the new spark plugs will burn a bigger hole in your wallet.
Lastly: if these so-called "firestone" spark plugs claimed to be as good as they can be, why EPA or any major automobile manufacturers haven't look into this yet? Don't they also wanna raise their CAFE as well?
My 2c's worth.