Multiboot FreeBSD 10.0 and Debian GNU/Linux using GRUB2

Debian GNU/Linux 8 Jessie Beta2 and FreeBSD 10.0 multiboot with grub2

Linux is on /dev/sda1
FreeBSD partition is on /dev/sda3 (ada0s3a - BSD style)
Grub is installed on master boot record.

Edit /etc/grub.d/40_custom and add the following lines:

menuentry "FreeBSD 10.0" --class freebsd --class bsd --class os {
insmod ufs2
insmod bsd
set root=(hd0,3)
kfreebsd /boot/kernel/kernel
kfreebsd_loadenv /boot/device.hints
set kFreeBSD.vfs.root.mountfrom=ufs:/dev/ada0s3a
set kFreeBSD.vfs.root.mountfrom.options=rw
}

Don't forget to run 'update-grub' after that and reboot.