Firefox includes the following bookmark folders by default:
Most Visited: Available from the Bookmarks toolbar, this folder lists pages you have visited the most number of times.
Recently Bookmarked: Available from the Bookmarks menu, this folder contains a list of the last several bookmarks you've added. Expand the Recently Bookmarked folder to see a list of sites.
Recent tags: Available from the Bookmarks menu, this folder keeps track of the bookmark "tags" you've used most recently. Expand the Recent Tags folder to see a list of tags, then expand each tag to show pages you've recently bookmarked using that tag. For more information on tags, see Bookmark Tags - Categorize bookmarks to make them easy to find.
If you have deleted one or more of these default "Smart Bookmarks" folders, you can restore them by doing the following:
In the address bar, type about:config and press EnterReturn.
The about:config "This might void your warranty!" warning page may appear. Click I'll be careful, I promise!I accept the risk! to continue to the about:config page.
Search for the preference browser.places.smartBookmarksVersion.
Double-click on browser.places.smartBookmarksVersion and change the value to 0 (zero).
Click the menu button
and then click ExitQuit
The next time you start Firefox, the default Smart Bookmarks folders will be re-created.