Firebug
I wrote about Firebug for Web Development in the past and can’t tell you how much I love it. Firebug integrates with Firefox to put a wealth of web development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.Download Firebug
Web Developer
I could go on for an hour about this extension. From the site; “The Web Developer extension adds a menu and a toolbar with various web developer tools.” I use this mainly for validating my site and testing for different screen resolutions.Download Web Developer
ColorZilla
This extension allows me to really get the color I need, EXACTLY. Advanced Eyedropper, ColorPicker, Page Zoomer and other colorful goodies…Download ColorZilla
MeasureIT
When dealing with WordPress themes you will find that you have limitations to the size of the photos you can use, but it is not always documented. With this extension I can find out exactly what size any element on my screen is. Draw out a ruler to get the pixel width and height of any elements on a webpage.Download MeasureIT
FireShot
FireShot is a Firefox extension that creates screenshots of web pages (entirely or just visible part).Unlike other extensions, this plugin provides a set of editing and annotation tools, which let users quickly modify web captures and insert text annotations and graphical annotations. Such functionality will be especially useful for web designers, testers and content reviewers.Download FireShot

