Firefox now always stores the old session, and you can access it by going to the History menu and selecting "Restore Previous Session"

If you want Firefox to display the message to save the session, it can be turned back on by changing some preferences.

1. Type about:config into the location bar and press enter
2. Accept the warning message that appears, you will be taken to a list of preferences
3. Locate the preference browser.tabs.warnOnClose, if its value is set to false, double-click on it to change its value to true
4. Repeat this for these 3 preferences browser.warnOnQuit, browser.warnOnRestart and browser.showQuitWarning

If you always open the last set of tabs, an alternative approach is this:

1. Click the orange Firefox button, then select options to open the options window
2. Go to the General panel
3. Change the setting "When Firefox starts" to "Show my windows and tabs from last time"

It was really helpful! Now everything is ok. Thank You!

I've done all this, and the problem persists.

If I open a new window, I get prompted when closing it, but my original window, using the session I had just prior to upgraded, still does not.

When I start up Firefox, I load the session I had open prior to upgrading. It never changes, no matter which tabs I close, add, or update prior to closing Firefox. Furthermore, the 'Restore Previous Session" option under History is greyed out.

Prior to upgrade I had Firefox set to always save without prompting.