Firefox keeps backups in the bookmarkbackups folder folder, so you can restore an earlier backup that looks hopeful. *https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them 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. You can look at this tool to inspect a compressed jsonlz4 bookmarks backup and possibly convert this file to HTML to be able to import/merge bookmarks. *https://www.jeffersonscher.com/res/bookbackreader.html