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 Recovering important data from an old 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/