That error message has several possible causes, for details see "Firefox is already running but is not responding" error - How to fix or Profile in use - MozillaZine Knowledge Base.

In your case I would first check to ensure that Firefox can find the new profile. In the second link above, see the section entitled "Check the profile folder name and location".

You need to use the Firefox Profile Manager to register that profile.

You can do that by creating a new profile and by choosing the location of that copied profile folder as the location of that new profile. Firefox will find and use existing profiles if you choose them.

See also http://kb.mozillazine.org/Recovering_a_missing_profile