The Professor X Extension let's you see inside a page's head without viewing the sourcecode.
Once installed the Professor X command is available by right-clicking as well as in the Tools menu. When applied to a page it will display the contents of the page's head element, including Meta, Script and Style content. This can help you see how a website was constructed without having to go back and forth between the sourcecode and the page in your browser.
This is the companion extension to X-Ray although either can be installed and used on their own. They were created to help people learning web design at various levels of experience.