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.