Firefox Developer Edition is a great and free browser with good tools.
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 Standard complaint
AFAIK, Firefox is the most HTML/CSS/JS standard complaint of all the browsers out there.
Pro Ready for work right out-of-the-box
It's Firefox. It's fast.
Pro Beta channel
As this branch of firefox uses a beta channel of firefox releases, you will see if future versions will contain breaking changes and give you time to react.
Pro Feature rich
This browser is the most feature richt browser for web development out of the box.
You can restrict the bandwidth in "Responsive Design Mode" to see how the page performs under unoptimal conditions.
It has predefined profiles for e.g. Ipad, Galaxy S9 and lots more.
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.
Con Not based on Chromium
If you don't want to make your website support non-Chromium browsers, this can be a problem.
Con This project is not yet available for Android
The developer edition is not available for mobile devices, though the beta and nightly channels are available, and can be used for testing future versions of Firefox.