Firefox keeps a backup of the bookmarks in the bookmarkbackups folder, so you can restore an earlier backup that has those bookmarks. *https://support.mozilla.org/en-US/kb/recover-lost-or-missing-bookmarks The name of automatically created JSON bookmarks backups 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 Library Restore menu drop-down list. *bookmarks-YYYY-MM-DD_<item count>_<hash>.jsonlz4.