This is the way that I create custom user directories. I usually do this before I install the program. Using Windows Explorer, create your directory, e.g. "NS Users." Then create subfolders in that directory for each profile that you will create later using the profile manager (the names you give to the subfolders will be the same names you will use later with the profile manager).
Now you are set. Install the program. Run the profile manager and create all of the custom profiles which you have set up under the user directory (you browse to the location of the new subfolders). Delete the default profile. You are finished.
Even though you have already installed NS, you can still make a custom directory with as many subfolders/profiles as you want. Or you can use your "Backup" directory, as you indicated. Be sure you use new profile names. Create your user directory and subfolders, as above (or create the subfolders in your backup directory). Use the profile manager to make the new profile (again, you browse to the location of the new subfolder(s)).
Once the new profile is made, copy and paste your prefs.js and user.js files, bookmarks (bookmarks.html), address books (files with the .mab extension), and mail folders from the old profile into the new profile. A note about mail folders. Only copy the mail files (inbox, sent, drafts, etc.)
without the .msf extension. Then launch the new profile and go through the preferences menu to be sure everything is set the way you want (including the path to the cache). After everything is working, you can delete the current active profile and the default profile using the profile manager.
If there is an easier way to accomplish the above, I'd sure like to learn about it. But it's the only way that has worked for me thus far.
Sisyphus
UserAgent: Mozilla/5.0 (Windows; U; WinNT; en; Stable) Gecko/20020911 Beonex/0.8.1-stable