My favorite new Firefox extension is Firebug (also see the Mozilla site.). If you do any web development or if you are ever currious how a web page works,  this extension will knock your socks off. It does way too much to cover here…okay I could certainly do it here, but I don't have the time just now! But in brief,  Firebug let's you inspect a web page in real time, even make edits! Want to see what happens if you change a font size? Change the positioning of a graphic? Change a CSS class name? No problem. Click and edit! Firebug lets you inspect HTML, CSS, JavaScript, the DOM, see the relative download sizes of images...in short it is a great inspecting and debugging tool! I don't know if I'd say it replaces DOM Inspector, View Source Chart Web Developer, or View Source Chart but it is shure a great addition to your toolchest! Here's a full list of my favorite Firefox extensions.