These screenshots are of the window generated by clicking on Bookmarks > Show All Bookmarks.
If you control-click on "Recently Bookmarked", and then click on "Delete" in the contextual menu, "Recently Bookmarked" will disappear...which is not to say that it won't reappear the next time you add a bookmark.
(I remember the "Recently Bookmarked" category having existed for quite a while.)
I assume you know that your Firefox is
way out of date?
