Cool - View Selection Source
Have you ever been interested in viewing just part of the source code that created a Web page? Perhaps there's one section of the page that you're curious about. Instead of viewing the entire HTML source, highlight the text of the area you are interested in, right-click (control-click on Mac) and then select "View Selection Source."

Not Cool - View Source Hits Server
Every time you view source in Firefox - whether using Firefox's built-in controls or alternate controls like those included in the Web Developer add-on - the view source window makes an additional server request to ensure you are not viewing source from your cache. While this initially may sound like a good thing it absolutely sucks if you are attempting to view source behind a certain session-enabled applications such as an intranet. This is the case for me at work. When I view source in our intranet Firefox makes an additional trip to the server where our session-management code sees the Firefox source window as a new session and shows the code prompting me to log-in again. To get around this feature I searched Firefox's add-ons and found the View Source Chart add-on. Not only does this get around the issue but it also produces a more readable document complete with color-coding and collapsible sections. Note, this add-on does not fix the problem if you are working in a frames environment. If you are interested in reading more about this add-on check out the authors Web site.

Cool - Refresh View Source Window
Another cool feature about viewing source is you can refresh the code, and hit the server again just like you would a normal Web page. Just hit control-R (command-R on Mac). This is pretty handy if you are making some code changes and you need to view the source to check your work.