Mozilla Thunderbird and SeaMonkey (and even the Mozilla Suite and Netscape 7) can actually be set up to use your Outlook Express address book, and/or your Outlook Contacts list. It’s just another one of those features that are not in the user interface.
To make Thunderbird use your Outlook Express address book, close Thunderbird, and add the following lines to your prefs.js file:
user_pref("ldap_2.servers.OE.description", "Outlook Express");
For Outlook Contacts, use these lines:
One important note: in order for it to work with Outlook, Outlook must be set as the system default mail client.
Interestingly enough, it appears this feature goes back to Mozilla 1.0.