In Webmail, click the Address Book icon to load the Address Book screen.
Click the Import/Export icon.
In the lower half of the screen (labelled Export Address Book) ensure that the format is set to Comma separated values and the source is set to My Addressbook.
Click the Export button, and save the file to disk.
Importing the address book into Thunderbird
In Thunderbird, from the Tools menu select Import.
From the dialogue box, select Address Books and click Next.
From the second screen, select the Text file option and click Next.
From the Select address book file window, ensure that the file type is set to Comma Separated (*.csv).
Locate the file and click the Open button.
In the final dialogue box, click OK to import your webmail address book.