1. Be sure you have following options checked in your Ubuntu kernel:
File systems -> Miscellaneous filesystems -> UFS file system support (read only) (UFS_FS)
File systems -> Partition Types -> PC BIOS (MSDOS partition tables) support (MSDOS_PARTITION) -> BSD disklabel (FreeBSD partition tables) support (BSD_DISKLABEL)
Default kernel for Ubuntu has already this options.

2. After rebooting type in console:

dmesg | grep bsd

You shoud receive similar output:

[ 2.108529] sda4: <bsd: sda8 sda9>

It means that you have Freebsd installed on sda4 partition, and sda8 and sda9 are so-called “slices”

3. Use command similar to:

mount -t ufs -r -o ufstype=ufs2 /dev/sda8 /media/free

For older versions of Freebsd:

mount -t ufs -r -o ufstype=44bsd /dev/sda8 /media/free

or

mount -t ufs -r -o ufstype=ufs /dev/sda8 /media/free

might be more suitable.