sqlite-manager | Sweeten your migrations when working with SQLite databases | Database library

NB: based on Firefox dev 63.0b6

Look at the data in moz_bookmarks more carefully. What is the fk in the row that has the tag title?

The real clue is in the doc [emphasis mine]:

moz_bookmarks: This table contains bookmarks, folders, separators and tags, and defines the hierarchy. The hierarchy is defined via the parent column, which points to the moz_bookmarks record which is the parent. The position column numbers each of the peers beneath a given parent starting with 0 and incrementing higher with each addition. The fk column provides the id number of the corresponding record in moz_places.

These tables are tightly referenced, so a join seems unnecessary. You might start with
FROM moz_places,moz_bookmarks tag, moz_bookmarks childs
and see where that takes you.

--ADDENDUM-->
You need two "types" of rows from moz_bookmarks that are mutually exclusive. The "tag" row, which has a title and an id, but fk is NULL. And the "child" rows from moz_bookmarks that have a parent_key which matches the id of the "tag" row and an fk (which is the id of the corresponding row in moz_places).

The key here is using a "self join". I found this tutorial that may help you learn this concept and build a working query. And in your case, the 'FROM' clause will include moz_places.

As regards the query posted in the comments: No rows will match because it explicitly selects rows with fk = NULL and there are NO ROWS in moz_places with id = NULL.