That's the places thing everybody talked so much about when announcing firefox 3. What we actually see in bookmarks or history is just a top of the iceberg. Had I read the places documentation before I started working on seoprofesional toolbar, I would probably just use the places tagging system or something to select the sites to remember...
Anyway, at the first attempt I just used favicon service as follows:
The trouble was, the icon was really the favicon from the site. The original size (whatever it was) and possibly animated. That I didn't like. So the second attempt looked like this:
The resulting icon is 16x16 and not animated. Only remember to put try...catch in the right places because you must close the container at the end (result.root.containerOpen = false)
(The last example was updated on the 7th of Feb 2010 to incorporate changes in FF3.6)