If I have multiple windows open - and click on firefox icon/tab on taskbar (when it is the active window) - firefox closes. It started this habit 2 days ago.
Go to Solution.
Try running Firefox in safe-mode.
If it works in safe-mode, that means one of your Add-on Extensions is causing the problem, which will require disabling them all, and then re-enabling them one by one to isolate the problem.
the add on causing the issue was minimise tray - an extension to down them all