Firefox Developer Edition is a great and free browser with good tools.
You can download it here: https://www.mozilla.org/en-US/firefox/developer/
Ranked in these QuestionsQuestion Ranking
Pro Ready for work right out-of-the-box
It's Firefox. It's fast.
Pro Can be launched alongside Firefox (release version)
Some might find that useless, but that's very practical to dedicate a browser to development.
Con May be unstable
Since it is a release channel between beta and Nightly, it may be unstable.