The following are some useful Firefox add-ons.

• Firebug - edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.
• Web Developer - has quite a bunch of web developer tools.
• Tamper Data - view and modify HTTP/HTTPS headers and post parameters.
• Live HTTP Headers - view HTTP headers of a page.
• ColorZilla - color picker.
• YSlow - analyze web pages based on Yahoo's rules for high performance web sites.
• ShowIP - show IP address of server serving the current page along with other IP tools.
• FireShot - create screenshots of web pages.
• Html Validator - add Html validation support.