When you are in the Firefox Organize Bookmarks (Library) window, you need to restore a JSON backup file, or Import a bookmarks.html file that was exported.

If you are looking to transfer your Firefox bookmarks to a different browser, you need to do an HTML export from Firefox, other browsers can't handle the JSON format for Firefox bookmark backups.

• Restoring a JSON file, replaces your current bookmarks.
  
• Importing an HTML file, adds to your current bookmarks. This can result in duplicate bookmarks. See:
  • https://addons.mozilla.org/en-US/firefox/addon/10897/
  • https://addons.mozilla.org/en-US/firefox/addon/1553/

Read the following carefully: http://kb.mozillazine.org/Backing_up_and_restoring_bookmarks_-_Firefox#Difference_between_restoring_and_importing_bookmarks