Had to be about a year ago - I pulled down the latest sources for gnu emacs and built them on my next slab running OpenStep 4.2 (I think? 3.3?) - I think I had to tweak one source file and I joked about how long it took to build ("this really feels like I'm getting something done!") but it worked. Not Firefox, obviously, but an impressive amount of backwards compatibility in that codebase.