After a clean install and starting FF without any initial changes (to get the new profile going) can't you then close out of FF and copy in the old profile? Or is that what you've been doing? It seems as if you have taken more steps than necessary.
As WindowStar mentioned, the new data will be in a folder similar to: Users>NewUser>AppData>Roaming>Mozilla>Firefox>Profiles>something.default
Copy the contents of your old user xxxx.default folder to the folder mentioned above and restart FF after also editing the profiles.ini and copying the Local folder files (described momentarily). I couldn't find any pathing changes that needed to be done as
WindowStar also mentioned. WidowStar's reference to "..\<username>\AppData\Mozilla\Firefox\Profiles.ini" should have read "..\<username>\AppData\Roaming\Mozilla\Firefox\Profiles.ini
Profiles.ini appears thus:
Path=Profiles/change to match the something folder name above for the new user.default
If you have set any additional profiles up they will appear below
You will also have to do the same thing for the folder in "
" but there will be no ini
file to edit.