[tmhedberg]

The Pentadactyl plugin for Firefox (http://dactyl.sourceforge.net/pentadactyl/), which is a better-maintained fork of the well-known Vimperator, allows you to hit <C-T> in any textarea or text input to enable Vim-style keybindings inline. <C-I> opens the text for editing in an actual Vim instance, like the plugin you mentioned.

Of course, Pentadactyl changes a lot about how the browser works beyond just text editing, so it may not be for everyone.

[mahcuz]

I can't live without pentadactyl.

[haridsv]

You can use jVi plugin that automatically enables vi bindings on text boxes.

[dredmorbius]

Nice. I swear by Vimperator myself.