Several months ago I came across some information on the BetaChurch.org website about a Firefox extension called Aardvark. It has been languishing in my Bloglines clippings folder since then but I finally got round to trying it today: what an excellent tool!
What it does is allow you to explore how a website is constructed without having to view the source code (Ctrl-U on Firefox) and pick through it to find the bit you’re interested in. Instead you can just hover over the page and read the pop-menus, exploring just the source code for the bit you’re interested in. Great for nosing around and even better as tool for troubleshooting little glitches on your own sites!