Also rewritten for Chrome & Chromium-based browsers by @nekman
Original author "csdev" :
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/stylesheet 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 "SS", "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".
With the latest version you can now view the embedded css and js as well (FF2 and SM2 only). Just open the "Page Info" window from the context menu or tools menu. There will be 3 new tabs in that window. One for scripts, one for stylesheets, and one for frames. You can open them by double clicking or by using the context menu.
You can also right click on the 'view page source' menu item and open the url of the current page with JSView. This allows you to open the source code of any web page in a new tab or in an external editor.