With Firefox, you can do much more than simply browse web pages. Pencil is a free add-in that lets anyone sketch complex GUI screens or web page prototypes in Firefox without even being a graphic designer.
Pencil includes a set of stencils that you can place on the drawing campus. Right click a shape to edit basic properties like color, border, opacity, etc. Alternatively, you may also drag images from the desktop.
Like other illustration software, these shapes can aligned with each other or can be grouped to prevent accidental edits. The drawing can be exported as a PNG image.