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The button basically copies the URL you're on and takes you to the PrintFriendly website to format/print it. This is good except there is an annoying bug regarding the button itself; where it basically changes its position upon restarting Firefox. If you remove the button from the bar altogether, it magically reappears at the bottom-right of the add-on bar when you restart.
I wouldn't normally post a bug as part of a review except I contacted PrintFriendly and they said they did not produce this add-on and therefore can't fix it (though are apparently working on their own official version).