These are the extensions that I must have after a new installation.
Protect your privacy. See who's tracking your web browsing and block them with Ghostery.
The Web Developer extension adds various web developer tools to the browser.
The User Agent Switcher extension adds a menu and a toolbar button to switch the user agent of a browser.
Advanced Eyedropper, Color Picker, Gradient Generator and other colorful goodies...
View HTTP headers of a page and while browsing.
Finds HTML elements matching chosen CSS selector(s) or XPath expression
Firecookie is an extension for Firebug that makes possible to view and manage cookies in your browser
Firecookie is part of Firebug since Firebug 1.10
Server Spy indicates what brand of HTTP server (e.g. Apache, IIS, etc.) runs on the visited sites.
Displays the contact photo/image/picture of the sender/recipient while reading or composing a message. Supports photos in a local folder and the 'Face' header for embedded photos. The add-on adds new generic photos and many styling possibilities.