All of a sudden I started receiving daily crontab email notifications from my web server with the following warning:

expiretable: Error opening "/dev/pf": No such file or directory

I checked my /etc/rc.conf and as expected it contained the following required lines:

$ vi /etc/rc.conf
pf_enable="YES"
pf_rules="/etc/pf.conf"
pflog_enable="YES"

So in theory everything should have started without any problems. Well, maybe not so just in case I fired off the following two commands:

$ kldload pf
$ /etc/rc.d/pf start
$ /etc/rc.d/pf status
No ALTQ support in kernel
ALTQ related functions disabled
Status: Enabled for 0 days 00:10:34           Debug: Urgent
...

Now working. If things go wrong again I'll have to investigate the problems better.