If you have accidentally removed bookmarks then use "Organize > Undo" in the bookmarks manager (Bookmarks > Organize Bookmarks) to get them back. The Organize button is the first of the three buttons on the toolbar in the Library (Bookmarks Manager). That only works if you haven't closed Firefox. Otherwise you need to restore the bookmarks from a JSON backup. That will replace all the current bookmarks and you lose bookmarks that were added since that backup was created. See http://kb.mozillazine.org/Backing_up_and_restoring_bookmarks_-_Firefox