Web Developer

All sorts of fun things.

Adblock

Lets you block ads.

Adblock Plus

Blocks ads, gets rules from some central lists.

SwitchProxy Tool

Lets you switch between multiple proxy configurations. Useful if you sometimes tunnel through an ssh socks proxy.

User Agent Switcher

Lets you lie to web servers and say that you're using a browser other than Firefox.

Firebug

JavaScript debugger. Also has cool browsing of an html document in multiple modes, live editing of style and content, etc.

Aardvark

Lets you tweak how a page looks one time only. Useful for preparing a page for printing.

X-Ray

Lets you see the html tags around the parts of a page. Useful for seeing how it's written.

Greasemonkey

Lets you write scripts that will change how particular web pages are displayed.

Platypus

Lets you create Greasemonkey scripts in a Wysiwyg fashion.

Tamper Data

Lets you tamper with requests before they are sent out.

Selenium IDE

Lets you record and playback macros for web interactions.