I am obliged to say I'm not a coder, but the idea might still be useful if it hasn't been implemented yet, if there is enough interest for it and if someone is willing to implement it.
The idea is a firefox extension which would allow you to make scribbles or quick notes over the web page you're viewing. You select any element, usually text, right click and choose to add a scribble and it gets added to the page by modifying its output to add a little icon after the selected element which reveals your scribble once clicked. Editing the page like this is possible as greasemonkey plugin proves, but such extension would be different from greasemonkey in that there is a specific use case for which it is designed for. I know I can edit any page with greasemonkey to add my notes, but it isn't very attractive when I have to write code to do it. A scribble extension would make this dead easy: right click, choose "add scribble", enter whatever text you wanted to and you're done.