A new feature called Reset Firefox gives you the ability to "reset" the browser while keeping your personal data. Currently available in Firefox 13 beta for Windows, Mac, and Linux, the feature works by migrating your bookmarks, passwords, cookies, and form data to a new default profile. Then it kills off the old one, and you've got a clean Firefox installation without having to troubleshoot anything.
It does not save add-ons, Sync setttings, open tabs, or tab groups, and it's not clear at this point if those are planned for Reset Firefox or if saving them would prevent it from working properly.
