As long as you didn't download any extensions with coreplus before you installed and booted corepure64, things should be OK - unless you specifically changed the name of the /tce folder, both core and corepure put extensions in the same place...
I don't use firefox, but opera12 works fine in corepore64 and the 64-bit versions of libpng, libtiff, libexpat, etc are all there in the corepure64 repo.