> Emacs cursor movement keystrokes are quite widely supported elsewhere too

Yes, even in Codex and Claude Code.

> Those work well also besides shells with Chromium/Chrome/Safari... My only gripe is that Firefox and its derivatives it doesn't work any more

Interesting, my experience is exactly the opposite: I had to finally bite the bullet and migrate to Firefox because Chrome/ium switched to GTK4 which removed key themes support.

(That's OK though, I should've moved off Chrome a long time ago.)