note:
Thunderbird supports multiple address books: the export address book functionality will export the contacts from the selected address book. To export contacts from other address books, repeat the procedure outlined below for each address book.
After having selected the address book you would like to export, go (from Thunderbird's address book window) to Tools > Export
The Export Address Book dialog will open, and let you pick a file name, a location and a file format for your exported contacts.
Thunderbird lets you export contacts from your address book in three formats: LDIF, for LDAP directories, CSV (Comma-Separated Values), the most compatible used format (where contacts are listed on each line, and contact information is separated by commas), or Tab Delimited.
The CSV format enables you for example, to import the contacts you exported from Thunderbird's address book into Microsoft Outlook or Outlook Express, even Excel with a single click.
I found saving the file and opening it in exel and import, this is the quickest way to work with mail merge.
Here is a tool it must be install into Thunderbird and restarted.
http://nic-nac-project.de/~kaosmos/ImportExportTools-2.3.3.xpi
Full info
http://nic-nac-project.de/~kaosmos/mboximport-en.html
hope it helps.
