Tags offer a convenient way to file a bookmark under several categories. You can tag a bookmark with as many terms as you want. each tag, an item is automatically created in the Recent Tags folder of the library. Drag and drop an item for a tag onto the bookmark toolbar to easily access it.
To add tags to a bookmark, open the bookmark in Firefox and click the yellow star in the location bar. The Edit This Bookmark dialog opens where you can add a comma separated list of tags. It is also possible to add tags via the bookmark's properties dialog which you can open in the library or by right-clicking a bookmark in the menu or the toolbar.