Web Developer Addon for Firefox

Couldn’t do without this tool, an absolutely necessity for modern Web Development. Seems to be updated quite regularly too, as it always works with new versions of FF (FireFox).

This extension adds a menu and a toolbar to the browser with various web developer tools.
It is designed for Firefox, Flock, Mozilla and Seamonkey, and will run on Windows, Mac OS X and Linux.

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. And there’s more…

Source: https://addons.mozilla.org/en-US/firefox/addon/271