Normally, windows will be re-opened automatically if they were "open" at the time you exited out of Firefox, and the last three "closed" windows will be listed on the History menu so you can open them manually. If you go to the History menu right now, are there any closed windows you can restore from your previous session?

To give Firefox the best chance of recognizing the windows you have open at shutdown, exit Firefox using the menu, either:

• "3-bar" menu button > "power" button
• (menu bar) File > Exit

Then Firefox will treat all windows open as of that time as open.

However, if you have Windows close Firefox by using the "red X" button on the title bar or right-clicking Firefox on the Task Bar and using "Close All Windows" or by simply shutting down Windows, then Windows will exit Firefox by sending sequential close window commands, and that could cause this issue.

The number "3" for closed windows to remember between sessions can be increased or decreased. To make sure it hasn't been reduced to zero, you can check here:

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

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

(3) The browser.sessionstore.max_windows_undo preference controls the number of windows you can re-open after closing them. To set a different value, double-click the preference and enter your preferred number of windows (I use 10) and click OK to save it.

As you might guess, the browser.sessionstore.max_tabs_undo preference controls the number of closed tabs you can re-open in each window.

Please be careful editing session-related settings, as you could break the feature if you don't research (or ask about) preferences before making changes.