Creating a new default profile
From MozillaZine Knowledge Base
Many problems with Firefox can be solved by creating a new Firefox profile. Sometimes you may have problems opening or using the Profile Manager or, in rare cases. any newly-created profile disappears from the Profile Manager, even though the Firefox profile folder itself is created. . To get around these problems or if you just want to start fresh, you can force Firefox to create a new default profile by exiting Firefox (if open) and then deleting or renaming one of the following, located in the default profile path:
For example, on Windows, the profiles.ini file would be located here:
The "Application Data" or "AppData\Roaming" folder is normally hidden. To open it and the contained folders inside, press "Windows key + R" to open the Run box (or click "Start → Run..." on Windows XP). Type in %APPDATA% in the Run box and click "OK". You can then open the "Mozilla" folder and then the "Firefox" folder, which contains the profiles.ini file.
In most cases you should rename the profiles.ini file (for example, to "profiles.iniOLD") OR the Firefox folder (e.g., FirefoxOLD) instead of deleting it, which lets you reverse your changes. When you delete or rename the profiles.ini file or the containing folder, the Profile Manager loses track of all existing Firefox profiles. When you restart Firefox, a new default profile folder will be created and only a single profile, the new "default", will be listed in the Profile Manager. This new "default" profile doesn't contain any of the profile data or settings stored in your old "default" profile folder, which still exists but is no longer accessible from the Profile Manager.
Warning: If you delete or rename the "Profiles" or profile folder by mistake, instead of the profiles.ini file or the containing "Firefox" folder, you'll see a "profile in use", "already running" or "profile cannot be loaded. It may be missing or inaccessible" error message when you restart Firefox (bug 278860). If this happens, go back and restore the folder you renamed or deleted, if possible, and then delete or rename the profiles.ini file or the containing "Firefox" folder. (Renaming a file or folder is always a safer option than deleting it.)
If you rename the profiles.ini file (e.g., to "profiles.iniOLD") instead of the containing folder, you may also want to rename the existing default profile folder (e.g., to xxxxxxxx.defaultOLD) so that you don't confuse it with the new default profile folder that will be created, which will also be named "xxxxxxxx.default" but with a different series of 8 random characters. However, if you later restore the original profiles.iniOLD file you will also need to rename the xxxxxxxx.defaultOLD profile folder back to the original name, so that Firefox is able to find the profile.
After you delete or rename the profiles.ini file or the containing folder, when you next start Firefox it will open with a new default profile, just as though you installed Firefox for the first time. Your existing Firefox user profile data, which includes bookmarks, passwords, cookies, preference settings and added extensions, will no longer appear in the new profile but you can transfer the data from the old profile folder to the new default profile folder.