Mounting UFS file systems in Ubuntu should be straight forward. Follow these simple steps to make sure that your are mounting your FreeNAS disks properly in Ubuntu:
Load the UFS module (or make sure its loaded)
sudo modprobe ufs
Make a directory where to mount the files on
To find out which device holds your UFS file system use the command
fdisk -l, note which device you intend to mount.
Mount the file system to the directory your previously created
sudo mount -r -t ufs -o ufstype=ufs2 /dev/sdb1 /home/<your_username>/ufs_mount
sdb1 is the UFS partition you want to mount and
<your_username> is the your current username where the intended mounting point was created previously.