Mozilla has introduced a new feature into the Firefox 13 betas called Reset Firefox which allows users to reset browser settings while retaining personal data.
The new feature is available in the Firefox 13 beta for Windows, Mac and the Linux platforms. Resetting the browser will creates a new user profile and migrate any bookmarks, passwords, cookies and form data to that new profile. Everything else gets set to the defaults, including add-ons, Sync settings, open tabs and tab groups.
Previously, in order to get a new profile, users had to manually copy 'places.sqlite' from their old profile to their new profile.
The first implementation of the 'Reset' function is a button on the Troubleshooting Information page (about:support).
The latest Firefox Beta is available for download here