Pretty awesome; I was looking at creating something similar, but saw it was already done. It can capture an entire webpage, including parts that are outside the visible window. It adds an icon; just click on that, and choose "Capture Whole Web Page." It creates an image, and then if you want to save it just click on the disk icon. The only problem I found was that often the "Capture" options are greyed out. That happened at addons.mozilla.org (which doesn't let any addons run), a local webpage from my computer, and an "about:" page. But it looks like normal webpages will save just fine.