Just when I think I’ve seen everything that you can do in Firefox, along comes an add-on like Pencil (Firefox 3 required). By coupling the Gecko drawing engine with the ability to display, save, and load an external canvas, together with a palette of shapes, Pencil installs an entire drawing application into your Firefox browser.
If you’ve worked with Visio or OmniGraffle, you’ll have the general idea: drag shapes to the canvas, move and resize them with the mouse, set their properties. There are basic shapes and a variety of UI widgets available. Pencil supports undo and redo, scaling, rotating, setting the z-order, adding external text and graphics, and so on. If nothing else, this means that you now have a tool for quickly prototyping new UI ideas wherever you have a browser handy.