Firefox Webpage Screenshot Add-On

I am creating a service for users of an online chat/virtual world that enables them to report abusive or malicious behaviors to an admin. The Firefox add-on should allow the user to click a button that immediately takes a screenshot of the active webpage and begin uploading the image to my web server. The chat/virtual world, however, operates by Java, and HTML canvas methods of taking screenshots will not capture the Java applet. Your task would involve creating a program or library in an XPCOM supported language (commonly C++) that will take screenshot of the active webpage, and send the image to a script that I can modify to handle the image throughout the Firefox add-on. The project must be packaged as a installable add-on (.xpi). When installed, it must allow me to click a button on Firefox's chrome that initiates the screenshot and displays it afterward.