Firebug
Firebug - web development evolved.
“Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.” - Firebug Site.
This is a great tool for all you web developers! It is great to be able to edit CSS, HTML, and JavaScript on the fly. This can potentially save some serious development time.
Some of the more advanced tools that stand out include: monitor network activity, debug and profile JavaScript and explore the DOM. I give this first edition of the Firefox add-on top marks; well worth having a look at this one!