After adding some entries to the system's hosts file, I can finally use the FF extension inside the Great Firewall of CN. It seems that no server is available without editing the hosts file.
Using it for a week, It's pretty fast & stable 👆. I can watch YouTube live in 1080p. But finding an English-speaking & fast server location takes much time.

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.

Since Version 3.1.0 it works realy perfect and looks good.

Take it from experience, forget the rest and go with the best. Surfshark is the best and well worth the investment especially in this day and age. Visit us at https://climatechangecommunity.com - Thanks!