Firefox keeps backups in the bookmarkbackups folder. *https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them Note that restoring a JSON backup replaces all current bookmarks, so you lose newer bookmarks that aren't in the backup. The name of an automatically created JSON bookmarks backup in the bookmarkbackups folder includes a total item count (folders and separators included) and an hash value to prevent saving the same backup more than once. You also see this count in the Restore menu drop-down list. *bookmarks-YYYY-MM-DD_<item count>_<hash>.jsonlz4.