>I wouldn't be surprised if the OpenBSD port used the system malloc

I thought so too but then when I looked for such changes, I couldn't find any.

    cvs -qd [email protected]:/cvs get -P ports/www/mozilla-firefox
    cd ports/www/mozilla-firefox
    grep -Ri malloc .

All that was found was a few irrelevant things which matched because "malloc" was in the name of the referenced files:

    ./patches/CVS/Entries:/patch-js_src_ctypes_libffi_src_dlmalloc_c/1.4/Tue Sep  2 16:43:04 2014//
    ./patches/patch-js_src_ctypes_libffi_src_dlmalloc_c:$OpenBSD: patch-js_src_ctypes_libffi_src_dlmalloc_c,v 1.4 2014/09/02 16:43:04 landry Exp $
    ./patches/patch-js_src_ctypes_libffi_src_dlmalloc_c:--- js/src/ctypes/libffi/src/dlmalloc.c.orig   Wed Jul 23 05:13:14 2014
    ./patches/patch-js_src_ctypes_libffi_src_dlmalloc_c:+++ js/src/ctypes/libffi/src/dlmalloc.c Thu Jul 24 20:47:22 2014