Also Windowmaker. I have been using it for years back in the day, and just to be sure reinstalled it a few minutes ago to check how it behaves today: no problems whatsoever with Firefox, LibreOffice, Gimp, Handbrake, and other modern apps. Using it right now to write this post with Firefox, in fact. Not that I didn't expect it to work since the last stable version is dated 2020 and the GitHub repo is quite active (last update 3 days ago).
Nostalgia aside (I was lucky enough to attend a real NeXT demonstration in 1994/1995) WindowMaker has been literally a life saver over 20 years ago when I had to build a desktop for ~50 remote points of sale in which the operator could only perform a few tasks, and setting up it as a locked "kiosk on steroids" with a fixed dock that could open only the 4-5 needed apps allowed them to do their jobs without distractions or exposing the unnecessary/dangerous parts of the system.