I haven't used org-mode, so I may be a bit unclear on exactly what is meant by folding editors. However, if we're talking about collapsing sections of text, I wonder if the oN-Line System (NLS) would qualify, as shown in Douglas Engelbart's 1968 "The Mother of All Demos" [1].

[1] https://www.youtube.com/watch?v=yJDv-zdhzMY&t=5m48s