You might like Firenvim or Wasavi:

https://github.com/glacambre/firenvim https://github.com/akahuku/wasavi

qutebrowser (and some others) also allow spawning vim from inside a text area, and reading the result back when the file is saved. I know Tridactyl can do it via its native messaging integration, but I suppose Vimium doesn't offer that.

This is supported by vimium with `gi`

> gi - focus the first text input on the page

No, that's not what I'm talking about. I want to be able to use shortcuts to navigate inside atextarea when I'm writing a lot of text.