All browsers include a "View Source" option, but none of them offer the
ability to view the source code of external files. Most websites store
their javascripts and style sheets in external files and then link to
them within a web page's source code. Previously if you wanted to view
the source code of an external javascript/css you would have to manually
look through the source code to find the url and then type that into
your browser.

Well now there's a much easier way. You can use the JSView extension to
solve the problem. You can access it from the context menu, from the
toolbar, from the view menu, or from the status bar. If the website you
are viewing contains any external js/css files, an icon will appear that
says "CSS", "JS", or both. Each individual file can then be viewed by
clicking on the filename. The file will be opened in a new window. You
can also choose to open all external files by clicking on "View All".

Author: Ron Beckman
WWW: https://addons.mozilla.org/en-US/firefox/addon/jsview/