Mephisto is a Mozilla Firefox 4 Add-On that adds an HTTP server inside your browser. This server is able to open webpages and returns content information or screenshots.
Common tools like curl or urllib2 will only give you the main HTML code of a webpage. On many websites, this source code is not the final document given to the user as some script may have modified it. Sometimes you want to check the validity of your document based on some criteria. Your source could be OK but your DOM a terrible mess.
Find more information on Mephisto website.