If you use Thunderbird and its contact functionality, you might already have stumbled over the „show on map“ feature. If you add addresses to your contacts (no matter if directly in Thunderbird or via CalDAV) there appears a button which enabled you to open a map with the contact’s location.
The default search provider is Google Maps. If you don’t like this service and prefer free and open systems like me, you can also add openstreetmap.org as your default map service. You only have to change a value in the advanced configuration.
Open the Settings menu, select the Advanced panel, select the General tab, and click Config Editor. Now copy in the search field:
mail.addr_book.mapit_url.format and double click on the string to edit it.
You should now see the default value. It’s nothing more than an URL with variables defined by the street, the country or the postal code of the respective contact. If you want to use Openstreetmap, fill in this value:
Restart Thunderbird and test again with an existing contact. By clicking on the button you should now see OSM instead of Google Maps where you can select one of the results in the left sidebar. Congratulations!