Importing Outlook contacts into the Thunderbird address book is quick and easy, but if you also want to transfer your contact groups into TB's mailing lists, there are some extra steps required. But if you have Contact Groups with thousands of entries, re-creating them im TB is probably not an option.
Start with any given Contact Group. Click File|Save As, choose Text Only (*.txt) as type and save that file to a location of your choice.
Start Excel and click Open|BrowseOpen the exported file
In the import wizard, make sure to pick the correct character set. Scroll in the Preview region to check that the national character set is chosen correctly. For the rest we can go with the defaults, so just click Finish.
Double check, that everything has been imported correctly. Then mark all the header lines and delete them.Click File|Save As, choose CSV UTF8 as output format and Save the file.Move over to Thunderbird/Betterbird and on the Address Book tab click Import.In the import wizard, make sure to pick the correct character set. Scroll in the Preview region to check that the national character set is chosen correctly. For the rest we can go with the defaults, so just click Finish.
Go with the default .csv and Continue
Uncheck First row contains field names. In my case, the defaults for the address book fields are OK, but you might need to adjust the choice, then Continue.I recommend to create a new directory. If something goes wrong, this is the easiest option to correct. Continue, Start Import and Finish. Now we have the Outlook Contact Group as new Address Book in Thunderbird.To get a Thunderbird Mailing List from these contacts, mark all and click New List.
Here we are, your Outlook Contact Group imported as a Thunderbird Mailing List.