https://support.mozilla.org/en-US/kb/categorizing-bookmarks-make-them-easy-to-find?redirectslug=Bookmark+Tags&redirectlocale=en-US

https://assets-prod.sumo.prod.webservices.mozgcp.net/media/uploads/gallery/images/2021-04-29-13-27-24-b5af60.png

It used to be there. When it was deleted?, Why? and How I restore it?

SOLVED: https://support.mozilla.org/en-US/questions/1445487