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 Designed specifically for web development
Is the only browser I know with features specifically oriented to aid web development out of the box.
Pro Can be launched alongside Firefox (release version)
Some might find that useless, but that's very practical to dedicate a browser to development.
Pro Ready for work right out-of-the-box
Pro Standard Complaint
AFAIK, Firefox is the most HTML/CSS/JS standard complaint of all the browsers out there.
It's Firefox. It's fast.
Since it has a lot of tools to support debugging and web development, it is considerably heavier than the standard version of Firefox, definitely I don't recommend it for standard web browsing.
Con May be unstable
Since it is a release channel between beta and Nightly, it may be unstable.
Con Lack of support for source maps in blobs
Still a bit buggy support for CSS source maps coming from blobs. The team is working on it though, and the fixes will eventually land in the production code.