Finally the Linux version for Firefox works. And it reconnects when restarting the browser. Perfect :)
I use surfshark for a year now and I am satisfied with it. The comments below which criticize that surfshark is not free.. Free VPNs are getting their money from spying on you and selling the data - paid VPNs like Surfshark are financed by subscription plans. I hope, Surfshark keeps their fair subscription model, since privacy is the main reason for using such a VPN.