Firebug

Firebug is a life saver when it comes to web development -- from introspecting your CSS to digging up details on AJAX calls, Firebug is there to help see what's going on under the covers.

This is one of the gems on my system. Whenever you have to create some AJAX interfaces, this tool comes in and supports you where ever you want. No matter if it is simple JavaScript debugging (with breakpoints) or stuff like modifying css, html or checking what messages have been send between browser and server. I love it ;)

This is a must have tool if you are doing anything from CSS + HTML to AJAX and security auditing.

This tool comes straight from heaven. They can take every tool away from me except firebug and vim. If they do that i am going on a strike.

Fire bug is a great Tool...

It helps me to understand how Comet and Polling are different in architecture and performance
[Read Hacking Gmail and Meebo]

I don't know how the web developers before Firebug did the things that did.... i just imagine a very hard and blind work! Firebug is the best tool!

Now that I teach high school students how to write webapps with AJAX, I see it wouldn't be possible without firebug.

I cry thinking about the years of web development I have done (and wasted) without firebug.

I agree, this tool is straight from the divine...

Firebug is my #1 web design tool for transitioning ideas and images into real site.

My favorite aspect of the tool is that it allows you to right click and inspect a section of a page you are interested in seeing the code for and it organizes the code for your viewing. Not only this, it allows you to edit the code live and see the changes immediately.

When it organizes code for viewing, it does not show you every line at once, it groups divs, paragraphs, etc for the best viewing. Alongside the page code it also lets you view the accompanying css (which is wonderful). Along with these features it highlights the section of the page you are viewing code for to avoid any mistakes.

Along with this, it has the console that allows you to monitor Ajax requests, etc, simply to find the kinks, etc that could be causing problems for your backend.

My only drawback at this point is that it doesn't play so very nice with Firefox. It does cause increases in memory consumption by Firefox but besides that it is absolutely wonderful.

This tool is so essential I'd have to say that any website developer who doesn't use it is making a mistake. Also check out IETester for something similar when testing on IE.

Firebug is a really great (probably the best) tool for every web developer.

But since upgrading 1.4.x along with Firefox 3.5, the performance has seriously suffered. Hope this will get better in the next versions.