Smart bookmarks are live bookmarks that don't just refer to particular sites but actually generate live lists of sites according to parameters you define. For example, you might have a smart bookmark that lists the 10 sites you visit most often, or the last 20 sites you've visited with a particular keyword in their title.
To create a smart bookmark, select 'Organize Bookmarks' from the Bookmarks menu. In the window that opens, select 'Bookmarks Menu' in the left pane, then click 'Organize' from the toolbar, then 'New Bookmark' in the drop-down menu. Give your smart bookmark a descriptive name, such as "10 Most Recent Bookmarks." In the 'Location' field, you will enter in a line of code telling the smart bookmark what to do. Mozilla's developer site includes a list of commands you can use in smart bookmarks.