One possibility is that some third party software has created or modified a user.js file with the instruction to restore your previous session windows and tabs at startup. To check for that, please see the section of the following help article about the user.js file: How to fix preferences that won't save.

Another possibility is that Firefox thinks it crashed the last time it shut down, and is doing a crash recovery. That doesn't usually happen every time, but here's a way to diagnose it: instead of having Firefox automatically restore the previous session after a crash, have it present a list of your previous session windows and tabs so you can choose which ones to open. Here's how you make that change:

(A) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(B) In the search box above the list, type or paste sess and pause while the list is filtered

(C) Double-click the browser.sessionstore.max_resumed_crashes preference and enter 0 (that's a zero) and click OK.

Note: Please don't customize any other sessionstore preferences to zero, or you might lose the ability to recover your closed windows and tabs when you actually want them.

If you start seeing that screen at startup, then we need to figure out why Firefox is crashing during shutdown.

You can check if you have a user.js file in the Firefox profile folder that sets the browser.sessionstore.resume_session_once pref to true.

• http://kb.mozillazine.org/browser.sessionstore.resume_session_once
• http://kb.mozillazine.org/Preferences_not_saved

Thank you so much for your helpful responses, jscher2000 and cor-el. I ruled out the possibility of a crash recovery issue. But I did find an errant line in my user.js file, and after deleting it, the problem has now gone. Regards :)