Taken from the Mozilla help file.
You can export a Mozilla address book if you later want to import it into another user profile, move it to another computer, or use it with another program that can import address books. You can export an address book to one of these file formats: Mozilla (.ldif), tab-delimited (.tab), comma-separated (.csv), or text (.txt) formats.
To export an address book, begin from the Address Book window:
1. Select the address book that you want to export.
2. Open the Tools menu, and choose Export.
3. In the Export Address Book dialog box, browse to the location where you want to save the address book file.
4. Choose the file format for the exported address book (.ldif, comma-separated, or tab-delimited).
5. Enter a name for the address book file. Be sure to include the appropriate file extension (.ldif, .csv, .tab, or .txt).
6. Click Save.