As a companion to my previous post I thought I would stay on the "essentials" theme and cover what I consider as a web developer the essential Firefox add-ons.
Seriously, this is best thing since sliced bread when it comes to web development. It enables you browse the source as it is generated, any JavaScript or CSS that was loaded by the browser and it shows all AJAX calls along with their corresponding responses. Definitely a must have.
As a web developer this tool bar comes in at a close second and is handy for generating source (making it searchable), disabling JavaScript/CSS/cookies for testing purposes as well as editing or adding cookies that are accessible by that particular web page.
I have found this useful when troubleshooting unusual behavior created by firewall privacy settings on a end users computer. Easy to install and highly customizable.
This is not a web development related add-on, but it is useful for bookmarking across multiple computers and platforms. Making this the #3 add-on I install with Firefox.