Firefox might be crashing during shutdown. In that case, it will attempt an automatic restore. You can set Firefox NOT to automatically restore after a crash and instead give you a screen offering a choice. This would help identify whether that is what is happening with your Firefox:
(A) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.
More info on about:config: Configuration Editor for Firefox. Please keep in mind that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.
(B) In the search box in the page, type or paste browser.sessionstore.max_resumed_crashes and pause while the list is filtered
(C) Double-click the preference to display an editing field, and change the value to 0 then press Enter or click the blue check mark button to save the change.
