Copy www-client/firefox at 18.0.1

I need to fix this for Prefix (specifically, to make it work without
jemalloc).
Copy dev-libs/jemalloc at 3.3.0

This seems to be necessary for Firefox.
Fix dev-libs/jemalloc-3.3.0 for prefix

Someone had accidentally doubled up the prefix support here.  The fix
is to just remove the double prefix (one in econf, one in "emake
install").
Keyword dev-libs/jemalloc-3.3.0 ~amd64

This seems to be stable for me!  This way I can actually install it on
my systems.