I tried to start Firefox on a fresh FreeBSD install, and nothing happened. Running it form xterm, I saw this:
process 0000: D-Bus library appears to be incorrectly set up; failed \ to read machine uuid: Failed to open "/etc/machine-id": No such file \ or directory See the manual page for dbus-uuidgen to correct this issue. Segmentation fault (core dumped)
I was about ready to write a bug report for dbus, before I realised I hadn't even started it. Derp!
To enable it in rc.conf and start:
# echo 'dbus_enable="YES"' >> /etc/rc.conf # service dbus start
# service dbus onestart
Now Firefox starts.