Facebook doesn't expose the data about who visits your profile or not. I had a look at the code of this extension—it seems like every time you visit someone else's profile, the extension records this to a shady URL (
http://profilevisitors.56kupdate.com/addons/profilevisitors/interface.php to be specific), the Facebook ID of the person whom you visited.
And if the other user installs this extension, it will fetch the data from that site (this URL to be specific:
http://profilevisitors.56kupdate.com/addons/profilevisitors/list.xml) and then report to that person that you had visited.
So this requires both the person to install the extension and leak personal info—for now just the Facebook ID, but considering it has access to all sites, don't be surprised if future updates start to steal information.
In short, it sorta does work, but requires both to install and leaks a whole lot of infomation. Don't install this.
Also, why in the world would you want to know who visited you?