How to import and export your address book in Thunderbird

Click Tools and select Address Book
Select the address book you want to export.
Then click Tools again in the Address Book and select Export... while keeping your address book selected.
Save the file somewhere convenient as an LDIF file with whatever filename you wish.
Transfer the LDIF file to the machine you wish to import to (via email, USB drive, etc).
If you are transfering your address book to a different computer, move to the new computer and retrieve the LDIF file (save it to somewhere convenient like the Desktop or My Documents).
Repeat Step 1 to get back into the Address book.
Select Tools again in the Address Book but this time select Import...
Select Address Books and click Next.
Select Text file (LDFI, .tab, .csv, .txt) and click Next; select the location of the LDIF when prompted.
Click Finished.