I am using Ubuntu.
When I launched FTB Launcher it said:
¨No usable authlib available. Please check your firewall rules and network connection. Can`t start MC without working authlib. Launch button will be disabled¨.
Reinstall java. Check distribution specific fixes for SSL certs.
I 2 times reinstalled java but its not helped
I have been having the same issue, in the debug log it says authlib good only to say its not available later.