Contacts from Thunderbird to eM Client

I’m trying to transfer from Thunderbird to eM Client. The email works fine, but how can I transfer all contacts. I tried it with local folders, but only a few contacts were transferred

You could try exporting the contacts from Thunderbird, then importing them into eM Client. Another option would be to sync your Thunderbird contacts online (like with your email provider), then you could sync those with eM Client as well. I think most people do that now so they have the same contacts on their phone and in their email clients.

Who is your email provider Martin?

Thank you for your answer Gary!
However, I stopped using eM Client because of the synchronisation problems with the Fastmail server. I just kept/keep using Thunderbird and will try eM Client occasionally to see whether the synchronisation problems have been solved.