Here’s a good Firefox addon which allows you to take screenshots of web pages, annotate them, edit the image and save the image to a file, send it to clipboard or even email it. This addon also allows you to capture long webpages even if the page scrolls on the browser window.

FireShot is a Firefox extension that creates screenshots of web pages. Unlike other extensions, this plugin provides a set of editing and annotation tools, which let users quickly modify captures and insert text and graphical annotations. Such functionality will be especially useful for web designers, testers and content reviewers. It’s possible to choose whether entire web page or only visible part of this page should be captured. Screenshots can be saved to disk (PNG, JPEG, BMP), copied to clipboard, e-mailed and sent to external editor for further processing.

Link: Firefox Add-ons: FireShot