This is the first of a series of posts listing and giving some details of the Firefox extensions I use at work and/or home. Some you may have already heard of; others, I believe, are more obscure.
Extensions for web developers:
Web Developer: This is probably the standard extension you'll find on any web developer's computer. Disable just about any part of the web page, play with CSS, forms, images, display various information about the page, outline the various parts of the page, and more.
Aardvark: I don't see this extension mentioned as often as it probably should be. This is a nice interactive way to work with CSS. Once the extension is turned on, the various parts of the web page that you move your mouse cursor over will be outlined with a red box and show a caption that reports the HTML element type as well as the class and id if they exist. From there you can do various things with the element such as remove it from the page, make it wider, or make it narrower.
LinkChecker: Checking for broken links on your pages can be tedious. This extension will highlight the links on a page with a particular color to tell you if the link is good, broken, or forwarded.
ColorZilla: Use this extension to get the RBG value and hex code of a color on a web page.
View Formatted Source: Sometimes the code you see with Firefox's view source option is difficult to read. I generally use this extension for it's color coding of HTML code (which will also allow you to view CSS information for the elements in a page), but it is also capable of letting you select a "block" in the page and just view the source for that block.
MeasureIt: Measure the height and width of any element on a web page.