We already know that, despite a few remaining problems, Mozilla is the best web browser available (well, we enlightened folk anyway ;-) ), but here's another very cool feature - debugging web sites.
I've messed with this capability a couple times in the past, just to see what exactly it does, but I never did anything more than just skim over the surface of it. Well, I recently came accross this site, which gives a VERY thorough tutorial and using the debugging and development capabilities of Mozilla to their fullest extent. It's most definitely worth reading for anyone that develops web pages.
The tutorial can be found ere:
http://gemal.dk/mozilla/mozdev.html