Hi. I write JavaScript bookmarklets, which are like extensions that you save as a Firefox bookmark, for my own use. This would be perfect for your project. The advantage of a bookmarklet over an extension is that it is easily portable between browsers. You just save it and import it like you would any other bookmark.
Although I would need to research how to take a screenshot of a page using JavaScript, this should be fairly easy to implement.
Thanks for considering my bid. I'm off from my day job through Monday, so we can get started right away if you like.