Most fabulous recently! Rated 4 out of 5 stars
The extension can turn tags into hierachical folders, thus keeping the feeling of old Bookmarks system while offering the flexibility of tags. The QuickSearch bar (à la Google Desktop) is also really fast and convenient. And the connection to Google database is not slow.
This extension beats built-in Firefox Bookmarks mechanism easily, and can compete well with the extension of del.icio.us Bookmarks! Hope such framework could be used for other bookmarking services soon!