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.
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
It’s a ruler, hehe. Useful for pixel perfect aligned items on a page.
Retrieve the color of any item on screen.
You can see how much a page weights up to great detail, also gives you tips on how to improve some components.
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.