Firefox on FreeBSD-CURRENT crashing with FEBE

[grahamperrin@momh167-gjp4-hpelitebook8570p-freebsd] ~% date ; uname -v
Tue 26 Dec 2017 17:34:18 GMT
FreeBSD 12.0-CURRENT #0 r320869: Mon Jul 10 13:57:55 UTC 2017     root@releng3.nyi.freebsd.org:/usr/obj/usr/src/sys/GENERIC
[grahamperrin@momh167-gjp4-hpelitebook8570p-freebsd] ~% firefox -p FEBE
mozilla::detail::MutexImpl::lock: pthread_mutex_lock failed: Resource deadlock avoided
Segmentation fault (core dumped)
[grahamperrin@momh167-gjp4-hpelitebook8570p-freebsd] ~% pkg info firefox
firefox-56.0.2_9,1
Name           : firefox
Version        : 56.0.2_9,1
Installed on   : Mon Nov 13 22:44:52 2017 GMT
Origin         : www/firefox
Architecture   : FreeBSD:12:amd64
Prefix         : /usr/local
Categories     : ipv6 www
Licenses       :
Maintainer     : gecko@FreeBSD.org
WWW            : http://www.mozilla.com/firefox
Comment        : Web browser based on the browser portion of Mozilla
Options        :
        ALSA           : on
        BUNDLED_CAIRO  : on
        CANBERRA       : off
        DBUS           : on
        DEBUG          : off
        DTRACE         : on
        FFMPEG         : on
        GCONF          : off
        INTEGER_SAMPLES: off
        JACK           : on
        LIBPROXY       : off
        OPTIMIZED_CFLAGS: on
        PGO            : off
        PROFILE        : on
        PULSEAUDIO     : on
        SNDIO          : off
        TEST           : off
Shared Libs required:
        libdbus-glib-1.so.2
        libgtk-x11-2.0.so.0
        libXext.so.6
        libpango-1.0.so.0
        libjpeg.so.8
        libpixman-1.so.0
        libsqlite3.so.0
        libcairo.so.2
        libgtk-3.so.0
        libffi.so.6
        libxcb-shm.so.0
        libX11-xcb.so.1
        libfreetype.so.6
        libcairo-gobject.so.2
        libatk-1.0.so.0
        libhunspell-1.6.so.0
        libevent-2.1.so.6
        libvpx.so.4
        libplc4.so
        libXcomposite.so.1
        libxcb.so.1
        libgdk_pixbuf-2.0.so.0
        libgio-2.0.so.0
        libssl3.so
        libXfixes.so.3
        libnss3.so
        libogg.so.0
        libv4l2.so.0
        libgobject-2.0.so.0
        libnssutil3.so
        libplds4.so
        libharfbuzz.so.0
        libstartup-notification-1.so.0
        libX11.so.6
        libdbus-1.so.3
        libXdamage.so.1
        libnspr4.so
        libXt.so.6
        libgraphite2.so.3
        libicuuc.so.60
        libpng16.so.16
        libicui18n.so.60
        libvorbis.so.0
        libglib-2.0.so.0
        libfontconfig.so.1
        libsmime3.so
        libgdk-x11-2.0.so.0
        libgdk-3.so.0
        libXrender.so.1
Annotations    :
        cpe            : cpe:2.3:a:mozilla:firefox:56.0.2:::::freebsd12:x64:9
        no_provide_shlib: yes
        repo_type      : binary
        repository     : FreeBSD
Flat size      : 168MiB
Description    :
Mozilla Firefox is a free and open source web browser descended from the
Mozilla Application Suite. It is small, fast and easy to use, and offers
many advanced features:

 o Popup Blocking
 o Tabbed Browsing
 o Live Bookmarks (ie. RSS)
 o Extensions
 o Themes
 o FastFind
 o Improved Security

WWW: http://www.mozilla.com/firefox
[grahamperrin@momh167-gjp4-hpelitebook8570p-freebsd] ~%