firefox audio / youtube crashes

[Gary Aitken]

Firefox 22.0,1 core dumps when attempting to play a youtube video;
the video loads and firefox immediately crashes when it starts playing.

$ firefox

(process:8337): GLib-CRITICAL **: g_slice_set_config: assertion `sys_page_size == 0' failed
Assertion failed: (wrote >= 0 && wrote == got), function alsa_refill_stream, file /usr/ports/www/firefox/work/mozilla-release/media/libcubeb/src/cubeb_alsa.c, line 319.
Abort trap: 6

9.1-RELEASE-p5 FreeBSD 9.1-RELEASE-p5 #0: Sat Jul 27 01:14:23 UTC 2013     root at amd64-builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC  amd64

$ident /usr/ports/www/firefox/Makefile
/usr/ports/www/firefox/Makefile:
     $FreeBSD: www/firefox/Makefile 321792 2013-06-26 11:01:34Z flo

make showconfig
===> The following configuration options are available for firefox-22.0,1:
     DBUS=on: D-Bus IPC system support
     DEBUG=off: Install debug symbols
     GCONF=off: GConf configuration backend support
     GIO=on: GIO for file I/O
     GNOMEUI=off: libgnomeui support module
     GNOMEVFS2=off: GnomeVFS2 (virtual file system) support
     GSTREAMER=off: Multimedia support via GStreamer
     LIBPROXY=off: Proxy support via libproxy
     LOGGING=on: Additional log messages
     OPTIMIZED_CFLAGS=off: Use extra compiler optimizations
     PGO=off: Use Profile-Guided Optimization
     WEBRTC=on: Web Real-Time Communication
====> Options available for the single AUDIO: you have to select exactly one of them
     ALSA=on: ALSA audio architecture support
     PULSEAUDIO=off: PulseAudio sound server support
===> Use 'make config' to modify these settings

Is it something I've screwed up, or the port, or ...?