In the current Firefox release the "Bookmark This Page" menu entry in the Bookmarks menu changes to "Edit This Bookmark" once you have bookmarked the page and the star on the Navigation Toolbar is blue highlighted. So do not get confused by the missing "Bookmark This Page" in the Bookmarks menu and seeing "Edit This Bookmark" instead if you try to bookmarks a page that is already bookmarked. You can make a extra copy of a bookmark by dragging the link to a folder in the bookmarks sidebar or by doing a copy and paste in the Bookmarks Manager (Library).