Here are some FireFox Add-ons I have found extremely useful while designing web applications. The description of each add-on has been taken from the the official page.
ColorZilla - With ColorZilla you can get a color reading from any point in your browser, quickly adjust this color and paste it into another program. You can Zoom the page you are viewing and measure distances between any two points on the page. The built-in palette browser allows choosing colors from pre-defined color sets and saving the most used colors in custom palettes. DOM spying features allow getting various information about DOM elements quickly and easily. A very useful FireFox extension indeed!
Pencil – A very handy add-on to create diagrams and GUI prototyping
ColorSuckr – Extracts colors from images and builds color schemes
Yet Another Window Resizer - Resize browser window to default screen resolutions
FireShot - A Firefox extension that creates screenshots of web pages (entirely or just visible part). You can also insert text annotations and graphical annotations to the screenshot.
MeasureIt - Draw out a ruler to get the pixel width and height of any elements on a webpage
Web Developer - Adds a menu and a toolbar with various web developer tools
and the last one which is a must have for every web developer/designer