The author has marked this style as obsolete.
The author of this extension has since removed the button in question.
The latest update to FirefoxMenuButtons came out, and he brought a friend. You know the little green guy! You love him, you hate him, Well... now you dont have to deal with him. Props to the author though. He's got a real nice site and has got a real nice forum set up. Worth a look, and you can always add the other icon on your toolbar later.
Enjoy! Peace
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}