Is there a Firefox add-on to use vim to edit textboxes?

or "How do I answer questions on SO in Firefox using gVim inside the textboxes?"

Solution 1:

It's All Text!

From the extension page:

At the bottom right corner of any edit box, a little edit button will appear. Click it. If this is the first time you've used "It's All Text!" then you will be asked to set your preferences, most importantly the editor.

The web page will pop up in your selected editor. When you save it, it'll refresh in the web page. Wait for the magic yellow glow that means that the radiation has taken effect!

Solution 2:

Vimperator makes Firefox act very much like VIM:

Vimperator is a free browser add-on for Firefox, which makes it look and behave like the Vim text editor. It has similar key bindings, and you could call it a modal web browser, as key bindings differ according to which mode you are in.

Once you have the cursor in a text box, hit Ctrl-I to open in your editor, which defaults to gvim.