Web Developer - Adds a menu and a toolbar with various web developer tools.
PDF Download - Firefox Extension - Allows to choose whether you want to view a PDF file inside the browser (as PDF or HTML) or you want to download it!
View formatted source - Displays formatted and color-coded source. You can fold/unfold/hilite block elements (table, tr, td, div, span,...). Really cool feature: select a block level element of interest directly in the page and view it's source! To help you to quickly analyze the source code you can view images directly from the source and add comments to folded block elements and to each single line. The code view is based on the rendered document, so you will also see dynamically (by Javascript) created and modified html elements! Works also with frames and selected text.
MeasureIt - Draw out a ruler to get the pixel width and height of any elements on a webpage.
Add N Edit Cookies - Cookie Editor that allows you add and edit "session" and saved cookies.
ColorZilla - Advanced Eyedropper, ColorPicker, Page Zoomer and other colorful goodies. 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.
DebugLogger - An extension developer's debug console that is a cleaner alternative to using dump(). It breaks up debug message to be unique per extension or project and let's you view them independantly in a clean colsole.
EditCSS - Stylesheet modifier in the Sidebar.
JavaScript Debugger - Venkman is the code name for Mozilla's JavaScript Debugger. Venkman aims to provide a powerful JavaScript debugging environment for Mozilla based browsers.