firefox 21.0: "Your Firefox profile cannot be loaded. It may be missing or inaccessible"

[Matthew Luckie]

Since I upgraded to firefox 21, I've been unable to start it.  It stops
loading with "Your Firefox profile cannot be loaded. It may be missing
or inaccessible".  I have tried shifting .mozilla out of the way (so
that firefox will create a new one) and that does not fix it.  When it
runs, firefox creates ~/.mozilla/firefox/ (but nothing else) and gives
the same error message.  I have tried cleaning everything in /usr/local/
and /var/db/ports/ and then rebuilding all ports but without any luck.
There must be something simple I've done wrong but I can't figure it out.

$ uname -a
FreeBSD mylar.luckie.org.nz 9.1-RELEASE-p3 FreeBSD 9.1-RELEASE-p3 #0:
Mon Apr 29 18:11:52 UTC 2013
root at amd64-builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC  i386

$ ident /usr/ports/www/firefox/Makefile
/usr/ports/www/firefox/Makefile:
     $FreeBSD: www/firefox/Makefile 318394 2013-05-17 17:43:21Z flo $

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