This seems to only happen on Linux when closing Firefox using "Quit" from the app menu bar at the top-left of the screen (GNOME). It does not happen when using "Quit" from Firefox's hamburger menu or using Ctrl-Q. Maybe this has always been the behavior of using the GNOME menue and is an unpleasant side effect of the way GNOME triggers an app to close? (Closes each window individually, or something?)
Bug 1858782 Comment 2 Edit History
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This seems to only happen on Linux when closing Firefox using "Quit" from the app menu bar at the top-left of the screen (GNOME). It does not happen when using "Quit" from Firefox's hamburger menu or using Ctrl-Q. Maybe this has always been the behavior of using the GNOME menu and is an unpleasant side effect of the way GNOME triggers an app to close? (Closes each window individually, or something?)