How to use Thunderbird with GUMail: Exporting Address Books
Start Thunderbird.
Click on the Address Book button.
This will bring up the Address Book window.
In your list of address books, click on the name of the address book you want to export.
Click on Tools, then click on Export.
This will bring up the Export window.
Beside Save in, click on the little down arrow, then click on Desktop.
In the File Name field, type a name for the address book (make sure it ends with .ldif).
Beside Save as type:, click on the down arrow, then click on LDIF.
Then click on the Save button.
Your address book file has now been saved to the Desktop. You can now import the address book on another computer. (You can transfer the file via a floppy disk or other storage media, or you can even send it as an e-mail attachment.)