hello ecicci, this is currently a problem in firefox when you first close all open firefox windows and then close the application afterwards - the bug should be addressed in a later version of firefox.

as a workaround in the meanwhile you could close firefox (through firefox > quit) while the browser window is still running or try this: enter about:config into the firefox location bar (confirm the info message in case it shows up) & search for the preference named browser.sessionstore.resume_from_crash. double-click it and change its value to false.

Thank you philipp. I will give it a try.