If you, in the address line, type '''about:config''' and press enter, you get to the internal settings of Firefox. Accept the warning, and in the long list you get, at the top in the Filter line, type: '''browser.places''' to get a much shorter list of settings. In this list, find the setting '''browser.places.smartBookmarksVersion''' - rightclick this setting, select Edit, set the number in the dialog to '''0''' (zero) and click OK. Now, restart the browser and the bookmark "Most visited" should be restored on your bookmark toolbar.

See also: [[Restore the default Smart Bookmarks Folders]]