I have a feeling this is a duplicate but I can't find it exactly.
When closing Firefox, the following windows stay open (unexpectedly), holding the firefox.exe process open with no real benefit (IMO):
1. Library window (Downloads, history, bookmarks): I'm hoping this gets properly integrated into a tab in future)
2. Addon windows: I found that the ublock logger is "allowed" to remain open (not sure the use case here) after all other Firefox windows are closed.
Interestingly, the Dev Tools window gets closed correctly when the main FF window is closed 👍