Martin beat me to it. Seems to be a cross platform problem, and I can confirm that it happens when using -no-remote on Win10 and Pop_OS! 19.04 on TB 60.8
Could the issue really be in Firefox as it is the common denominator? -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/711305 Title: Thunderbird is already running, but is not responding. To open a new window, you must first close the existing Thunderbird process, or restart your system. Error received when clicking a mailto link in firefox. To manage notifications about this bug go to: https://bugs.launchpad.net/thunderbird/+bug/711305/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs