User blog comment:MisterWoodhouse/Social Activity, the 2010 Editor, and Unpausing Migrations/@comment-34918040-20201012190602/@comment-39885705-20201012231645

I think I get it now. You have the newest Firefox, but I still use version 56.0 (the new one isn't disability-accessible). Back in 2010, this editor was probably designed for Firefox 56. So it works great for me and crap for everyone else.

That said, the current Wikipedia editor works just fine for me, although it lacks syntax highlighting.