Comment 27 for bug 867424

In the most recent version (example: Firefox 63.0.3 on Cosmic) the behaviour is different. Firefox actually blocks the restart/shutdown process by showing a popup "X tabs will be closed". But the whole OS also creates a modal dialog box, asking whether to cancel, reboot or turn off, and the modal dialog boxes actually prevents the user from closing Firefox properly using the Firefox pop-up window. If "reboot" or "turn off" are chosen, Firefox will show the dreaded "Well, This Is Embarrassing" screen.

This is an improvement from the previous versions. However, a proper shutdown sequence should allow the user to close applications properly right after initiating the reboot/shutdown procedure.