If you are web developer you should already have installed Firebug because it makes life so much easier for the DOM/CSS/JavaScript programmer. Get it now (I mean it!) from addons.mozilla.org.
Now Joe Hewitt is bringing us even more goodness with the new 0.3 release of Firebug. It looks a lot fresher and it has a slew of features which are going to make stuff even easier. Brief rundown:
“View Source Live” — I used this one today to debug the placement of the Plazes badge on my site. You include some Javascript, and it then mucks around with your DOM. Without a ‘live’ source view, you can't see what is going on.
Events Inspector — Life saver for when you have to debug a particularly troublesome sequence of nasty events.
Dom Editing — This one looks really really killer! Always wanted to muck around with pages after they have loaded.
Hover Outlining — Mousing over DOM elements and CSS selectors in Firebug will outline them all on the page. Just like the Web Developer extension but much more flexible.
I ran it around the block today for a small debugging session and most of the new features work out well and are easy to use. It almost makes me feel sorry that my work is no longer focused on pure web development. Where has this extension been all my life?