need firefox extension help

I am trying to make an extension that takes text out of an html input field and replaces it with some other text. I want to trigger this with a key press. I have been following along with this example, but am now stuck. https://developer.mozilla.org/en/XUL_Tutorial/Keyboard_Shortcuts#Assigning_a_keyboard_shortcut_on_a_menu

Using the code there, I should be able to press ctrl+v and get the alert pop-up. The alert only comes up if I don't have the cursor on an input field. Obviously, I need this to work when the cursor is an input field. Any idea how to fix this?

Conkeror.