As a web developer I frequently use Firefox extensions to increase my productivity.  From getting the pixel color of an image to checking the width of an advertisement, Firefox has an extension for anything you need to accomplish.  Here’s a list of very useful Firefox extensions for web designers.

Measure it

This is a very useful plugin that’s used to measure the height and width of an object in pixels.  Just click the ruler to give you an on screen ruler.

ColorZilla

Do you ever wonder what color a certain pixel is?  Do you want to match an advertisement to your content?  ColorZilla will give the hex code for any pixel on your screen.

SEOquake

This extension is a must have for webmasters that are link building.  It analyzes the quality of a page by getting the keyword density, backlinks, Facebook likes, and PR among many other things.  You can also have custom Google and Yahoo searches to sort Google results by things like page rank.
IETab

A lot of websites look different in Firefoxcompared to IE.  With IEtab you can open a tab in Firefox that is rendered by the Internet Explorer engine.

Web Developer

This is a very useful extension for CSS editing done within Firefox.  It has many useful features and is a must have for web designers.

LifeHTTPHeaders

This extension allows you to see the data that is being requested and sent using HTTP headers.  You can also edit it on the fly without resubmitting a form.

UserAgentSwitcher

This allows you to test PHP user agent code.  You can change your user agent to that of any browser with this useful extension.