The singularly most useful Firefox extension there is.

I have found coda an excellent coding environment. It is very clean and uncluttered and I enjoy using it.

Really light and simple code editor; bundles for a plethora of applications.

Simple editor with good syntax highlighting

Can't design/develop without it.

or CVS. Versioning systems are an essential tool to have!

Run up virtual machines from your desktop for browser testing.

Aptana Studio is the premier IDE for Today's Web offering integrated language support for HTML, DOM, JavaScript and CSS, embedded development platforms and databases for PHP, Jaxer, Ruby on Rails, and Python. Plus support for emerging platforms that use Ajax like Adobe AIR and Apple iPhone. Aptana Studio is free, open source software based on the Eclipse tools platform.

It's no Firebug, but still very handy

Mozilla's highly extensible web browser is invaluable when creating web sites.

Aww come on I think we all know how usefull this is to us!

Still a good software but I'm using it a lot lesser than a few years back.

simple, small notebook for sketches, ideas, & notes.

WebKit alternative to Firebug.

xScope is a powerful set of tools that are ideal for measuring, aligning and inspecting on-screen graphics and layouts. Quickly available via the Mac OS X menu bar, xScope's flexible tools float above desktop windows and UI elements making measuring a breeze.

The ultimate validation extension for Firefox. Shows validation errors on a site in real time, even when browsing, and suggests how to fix it. No need for an external validation site or update and check, update and check validation!

http://users.skynet.be/mgueury/mozilla/

"HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla. The number of errors of a HTML page is seen on the form of an icon in the status bar when browsing. The details of the errors are seen when looking the HTML source of the page."