Tridactyl is probably what you're looking for nowadays, assuming you're willing to use Firefox.
It's the spiritual successor to the likes of Pentadactyl and Vimperator, which are not available for the current version of Firefox.
If you want something more like It's All Text, where the editing area appears right on top of the browser text area rather than launching an editor window, and you're willing to use Neovim, check out firenvim.