I guess he used installer provided on mozilla.org, it looks like a shell script, basically removing the directory where mozilla was placed will suffice, suppose it was place under /usr/local/lib/mozilla, so
rm -rf /usr/local/lib/mozilla will be enough, I dn't think there are any other locations where the files were scattered, it's basically kept under one location
try as root
find / -type d -name mozilla -print
and it should give you an idea where it is.