In order to have ThunderBird read the contents of your /var/mail folder, you need to create a symbolic link to it.
this can be done graphically or via command line.
**** Command Line ****
Step 1.) Open a terminal
Step 2.) Navigate to the location you wish to have the folder displayed
- ie cd ~/.mozilla-thunderbird/[unique hexidecimal number].default/Mail/Local\ Folders/
- ***NOTE**** Your system will have a different .default folder. Tab-Autocomplete is very helpful.
Step 3.) Type ln -s /var/mail/[user-account]
This will create a symbolic link for the folder [user-account] in your Local Folder directory in ThunderBird
***Option**** If you want the folder to be displayed with a different name, for example your [user-account]=JoeSnuffy and you want the folder in ThunderBird to say "System" (or something similar), simply add the new folder name at the end of the above command (ie ln -s /var/mail/JoeSnuffy System )
Step 4.) Open or Restart ThunderBird.
**** Graphically ****
Step 1.) Open your prefered file browser (Natilus, Thunar, Konkorer, Rox-filer, etc)
Step 2.) Navigate to /var/mail/
Step 3.) Open a second instance of your prefered file browser (you may need to enable "Show hidden files" for your file browser
Step 4.) Navigate into the .mozilla-thunderbird folder of your home directory until you get to the sub-directory you want to hold your local mail
Step 5.) Use the "create symbolic link" method for your file browser (usually right-click, drag, create link) from the first instance to the second instance.
Step 6.) Open or Restart ThunderBird.