Firefox Extensions Essentials : 1 - Web Developer

Adds a menu and a toolbar with various web developer tools.
An essential extension for any web developer/designer as it provides a raft of incredibly useful features all under one roof. You will wonder how you ever managed without it!

The Web Developer extension adds a menu and a toolbar to the browser with various web developer tools.


It is designed for Firefox, Flock and Mozilla and will run on any platform that these browsers support including Windows, Mac OSX and Linux.


Features

Disable
Cache
Cookies
Image animations
Images
Java
Javascript
Page colors
Popup blockers
Referer Logging
Stylesheet of your choice
CSS
Add user stylesheet
Disable browser default
Display by media type
Edit CSS
Use border box model
View style sheet and infos
Forms
Clear radio buttons
Convert GETs to POSTs
Convert POSTs to GETs
Display form details
Enable autocompletion
Make fields writable
Populate fields
Remove max length
Show password fields content
Display form info
Images
Display image dimensions
Display file size
Display file path
Find broken images
Hide backgrounds
Hide images
Make images invisible
Outline images with :
Adjusted dimensions
Empty 'alt' attribute
Without 'alt' attributes
Without dimensions
Without title
Replace images with alt value
View image info
Information
Display access keys
Display anchors
Display block size
Display ID and class detail
Display link paths
Display tab index
Display topographic info
View cookie info
View javascript
View link info

http://releases.mozilla.org/pub/mozilla.org/extensions/web_developer/web_developer-0.9.4-fx+fl.xpi

Yes, I use this extension often. It is good for quickly seeing how the current site was made. I also think you can put your own CSS sheet onto a site for you, and make the site look how you want it to look.I also use it to check out if I correctly coded my sites, by going on the site I want to validate, clicking on tools, and then checking which you want to validate. It then automatically links to W3C and shows all your errors and warnings.

Yeah Web Developer's an invaluable tool. Probably it's best feature is being able to edit CSS right inside your browser and being able to see the changes you've made take effect instantly.