I use the Firefox browser alot for testing and developing websites, specially because there’s a few plugins out there that can help make web development and troubleshooting problems in web sites/apps.
Here’s some of them in no particular order.

Aardvark

Enables you to see and manipulate html and css elements on the websites. Essential for me. Some useful commands:
c - colorize a selection
w - expand selection
v - see code

MeasureIt
It’s a ruler, hehe. Useful for pixel perfect aligned items on a page.

Color Picker

Retrieve the color of any item on screen.

Developer Toolbar

Swiss army knife of plugins, allows you to see css and html code, turn off images, javascript, validate locals and online pages and much more. Just download.

YSlow

You can see how much a page weights up to great detail, also gives you tips on how to improve some components.

Firebug

Now you can debug that nasty javascript which breaks your page.

SEO Toolbar for Firefox

In short gives you a series of tools to assert the popularity of a site, from google, alexa, etc and helps you see how search engines interpret your site, or not.