This is a simple step to migrate thunderbird mails from Mac OSX to Ubuntu.
Create the following directories in Ubuntu Desktop
Mount the mac HD
mount -t hfsplus /dev/sda2 /media/mac
Copy the data from mac homedir
cp -rpf /media/mac/Users/home/rayber/Library/Thunderbird/* .thunderbird/
You are now good to go. Start thunderbird and you should be able to start from the place where you stopped in Mac.