Firebug integrates with Firefox to put a wealth of web development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.

Seoquake has been developed for the look-see of site's parameters in search results of Google, Yahoo, MSN, Yandex (russian SE), Rambler (russian SE) search engines and any other pages as well (Seobar)

The Web Developer extension adds a menu and a toolbar to the browser with various web developer tools. It is designed for Firefox, Flock, Mozilla and Seamonkey, and will run on any platform that these browsers support including Windows, Mac OS X and Linux.

Toolbar displays PageRank has Google Search for firefox

del.icio.us is a social bookmarks manager. It allows you to easily add sites you like to your personal collection of links, to categorize those sites with keywords, and to share your collection not only between your own browsers and machines, but also with others.

Toolbar for Yahoo Search for firefox

DL the Mozdev Googlebar and add the PR bar for both Mozilla and Firefox.

SEO for Firefox is a free Firefox extension which adds many search engine optimization data points into Google's SERPs and Yahoo!'s SERPs on the fly.

SEO Form is a Firefox extension to help speed up manual directory submissions. I regularly do directory submissions for my sites, and I was finding the whole process a bit painful having to re-enter the same information on every page. So I then had all my data in a text file open on another screen, and would copy-paste my data into the form fields one by one. This works ok, but is still a bit slow.