[psacawa]

Please check out firenvim, which accomplishes this very useful trick in a site-configurable manner and is compatible with Tridactyl.

https://github.com/glacambre/firenvim

[exikyut]

That only replaces <textarea>s. Theoretically the OP project would also work in <div contenteditable>s.

[OJFord]

And (more significantly surely?) outside the browser.