In my bookmarks I have over 800 (I use it over months) entries, I use every day over 20 newspaper from the bookmark toolbar and so I have 20 more high frequented bookmarks when I move the mouse on the right side the FF and I can have on the left side my RSS feeds.
I haven't tried this, but -- for those who like it can want Firefox 3, do you know you can put a folder on your regular Firefox bookmark toolbar? Or even several folders, with folders inside of them. Don't make too many or Firefox loads slower, but you can put several folders of frequently visited sites (plus the ones on the bookmark toolbar itself -- and you can fit a lot of them on the toolbar if you abbreviate the names that go with the icons). This seems very similar to what the description of this add-on says it does. I hope this helps someone. I'll check back and see if this one gets updated for 3.0 but since there's a note saying that might not be for awhile I thought my comment might help someone in the meantime.