In Firefox the bookmark backup files are in JSON format, which doesn't allow that backup file to be merged with existing bookmarks. Restoring a backup would overwrite your existing bookmarks. But if you would '''export''' your existing bookmarks in HTML format, you could '''import''' that HTML file to append / merge after restoring the backup. https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them#w_restoring-from-backups