Firefox will automatically restore your previous session if it thinks it crashed (i.e., it did not shut down correctly). To see whether that is happening, you can change a setting so that instead of automatically restoring the session, Firefox display a list of windows and tabs. If you see that, it's a sure sign that Firefox did not shut down correctly and we can focus on figuring out why that's happening. Here's how to make the change: (1) In a new tab, type or paste '''about:config''' in the address bar and press Enter. Click the button promising to be careful. (2) In the search box in that tab, type or paste '''sessions''' and pause while the list is filtered (3) Double-click '''browser.sessionstore.max_resumed_crashes''' and change the 1 to a 0 (a zero) and OK that. (4) You can close this tab now.