Enable Write File Single User FreeBSD

Boot to single user mode then:

# fsck
# mount -u /
# mount -a -t ufs

Now you should be able to edit /etc/rc.conf.

With # fsck you check the filesystems, # mount -u / mounts the root partition read/write, # mount -a -t ufs mounts all UFS partitions, so you can use vi and other tools in /usr.