The easiest way is to set up address book synchronisation and drag your contacts with the mouse from Thunderbird to your Posteo address book.
There is another way (for a large number of addresses) that functions using a special export plug-in, which you have to install for Thunderbird.
Right-click this link and choose “Save as”. Download the file, in order to install the plug-in in Thunderbird via “Extras” → “Add-ons”.
Now you are able to export the address book as a “vCard”: Go into your address book and right-click on the desired address book. Choose “Export as vcf file”. This file can then be imported into the Posteo webmail address book.