Good and Working (up to nightly[11]), but ... Rated 4 out of 5 stars

It works, but only on static links. Any links inserted via JavaScript are ignored. This is caused by the way you insert your listener.
see https://developer.mozilla.org/en/XPCOM_Interface_Reference/nsIXULBrowserWindow#setOverLink%28%29
for a better way to get the link the mouse is over.