More info - if I close Firefox I still cannot start Thunderbird. If I reboot and start Thunderbird first it works. If I open Firefox it still works but if I close Thunderbird I cannot restart it unless I reboot!

This doesn't seem to be a problem with VPN. I just noticed it after the VPN installation. The problem is with Thunderbird and restarting it only works with a reboot. Workaround don't close Thunderbird!

Starting thunderbird should not be affected by a VPN, nor Firefox.

Try starting Thunderbird

• in troubleshoot mode https://support.mozilla.org/en-US/kb/troubleshoot-mode-thunderbird
• or offline using command line "--offline" switch