Advanced Eyedropper, Color Picker, Gradient Generator and other colorful goodies...
Draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.
CodeBurner is a Firefox add-on that integrates with Firebug, to extend it with reference material for HTML and CSS.
Edit textareas using an external editor, because it's all text!
Right click on a textarea, select "It's All Text!" and edit the text in the editor of your choice.
Alternatively, click on the edit buttons added for your convenience. Right click on ...