I wouldn't be surprised if it's a vehicle for malicious code.
Here are screenshots of it, and I provided a link* for the curious, but I recommend that you NOT use it:
(*and if you check out the page of that link, you'll see that I'm only one of the four people who "Like"d it on the Facebook widget on that page. Unfortunately, there still isn't an "Unlike" or "Dislike" button.)
|ABOVE: fbbutton is listed among the toolbar menus of Firefox|
|ABOVE: fbbutton adds a toolbar to your Firefox browser|
|ABOVE: fbbutton options|
|ABOVE: It's easy to hit that "Disable" button. "Uninstall" isn't bad either. I only disabled it, kept it there for "archiving"|