Firefox Developer Edition integrates all the developer tools you'll need into one package. It is the only browser dedicated to developers.

How do I switch between the Developer Edition and standard themes?

Click the menu button , click More Tools… and choose Customize Toolbar… and choose Customize… Customize Toolbar…. At the bottom of the Customize panel, click on Themes, select a different theme and click Done.

Recover bookmarks and other data

Firefox Developer Edition will create a new profile to store bookmarks and other user data. See Restore bookmarks, passwords and data from an old Firefox profile for ways to copy this data from another Firefox profile.

How do I learn more about Firefox Developer Edition?

See https://www.mozilla.org/firefox/developer/