Closing Thunderbird
When I try to open Thunderbird I get message that it already running and must close it before getting my messages. Cannot close the supposedly running Thunderbird. What is going on? How to fix without Terminal. I am a newbie.
You can open the System Monitor, find the thunderburd process, and kill it.
Alternatively, run pkill thunderbird in a terminal window.
To actually fix it, you'd need to analyse and debug the problem, possibly edit the source code, and recompile. It will take a lot of time, so you probably want to skip it for now.