Mandatorily on all platforms, or just by default? Because of the security & debugging features in OpenBSD's malloc framework[0] I wouldn't be surprised if the OpenBSD port used the system malloc, I'm guessing that's just a compile-time switch away.

[0] http://man.openbsd.org/OpenBSD-current/man5/malloc.conf.5

>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

By "as developed by Mozilla", I meant that OpenBSD's non-jemalloc config is their own doing.

It can be configured to use the system malloc (IIRC running stuff like asan requires this, or at least I have to set a 'no jemalloc' build option), but I'm not sure if any platform ships with this option turned on. OpenBSD sounds like one that might, but I guess I thought they just used the normal Firefox ESR (definitely could be wrong though).

Perhaps you can check about:buildconfig or about:support for malloc type.

I don't think this is the case on OpenBSD.

That's my point: They use their own malloc that Mozilla doesn't test with, so slowness there is more attributable to OpenBSD than Firefox.