Developing any site, firefox extensions has been served a lot. Here is some useful firefox extensions that will reduce your work a lot

FireBug 1.3 : Edit, debug, and monitor CSS, HTML, and JavaScript live. (download)

Web Developer’s Toolbar: Adds a menu and a toolbar with various developer tools. (download)

View Formatted Source: Displays formatted source and CSS info for each element. (download)

YSlow: Analyzes pages and tells why they’re slow. REQUIRES FIREBUG. (download)

Dust-Me Selectors 2.11: Dust-Me Selectors finds unused CSS selectors in your site. (download)

UrlParams: Displays the GET and POST parameters of a web page. (download)

Link Checker: Checks all links on a webpage and reports broken ones. (download)