and the location of the folder is:
/home/you/.thunderbird/ or ~/.thunderbird directory.
All you have to do is backup this directory.
Go to Terminal (CTRL + ALT + T)
You should be located in the folder /home/you/ indicated by ~$ sign
Type cd .thunderbird to enter the folder and explore the subfolders. One of them contains the file with your Inbox mail. Make sure it is there.
Go back to with cd ..
Type cp -avr .thunderbird /media/your USB or hard drive name/
And you are done!
To restore Thunderbird mail and profile
Thunderbird should not be running. Simply copy backup files to your /home/you/.thunderbird/ directory.