How to bring back "Save and Quit" feature in Firefox 63+?

I've always enabled the browser.showQuitWarning feature in Firefox so I can choose to keep the tabs or not when I close the browser

However last week my Firefox updated to the version 63 and suddenly that option doesn't work anymore. I'm only asked to close tabs or not instead of "Save and Quit"

Is there any way to restore the old behavior? I don't want to enable Restore previous session since I don't always want to save the session

Mozilla did something sneaky with Firefox. It no longer has "Save and Quit", but it does the same thing.

• The interface isn't designed to choose saving tabs at quit time on a session-by-session basis. If you normally want to continue where you left off, you can use about:preferences (or Edit | Preferences) to set Restore previous session under General | Startup. That will always save the session and start the next session where you left off. You can also pin selected tabs so you close everything, but start with the pinned tabs.

• If you choose Restore previous session, Firefox will always close immediately via any close route. You will never get a warning, regardless of any settings in about:config.

  If you don't select Restore previous session, you will always get a warning if you try to close with multiple tabs or windows open. In V63, they removed browser.showQuitWarning in about:config; it's now automatic.

• The warning is just a reminder. There is no Save option; you can either quit or remain in the session.

• If you quit, your next session opens with just your Start tab or Home page, whatever you've set up, plus any pinned tabs.

• Here's the sneaky part. The session saving happens automatically in the background, regardless of the Restore previous session setting; that setting just lets you quit immediately, without the extra dialog window, and determines what the session looks like when you open it next time.

  If you don't use that setting and want to restore the previous session, open History. There is a one-click option to Restore Previous Session. So if sometimes you want it and sometimes you don't, this lets you restore it with just two clicks.

  This feature was actually available in earlier versions; there were just a lot of extra settings available to make choices more flexible and intuitive for different users; these were simplified in V63.

  Note that if you start a fresh new session, open a bunch of tabs, and then decide that you want to restore the previous session, you can. It will open the previous session tabs in addition to what you already have open, rather than replacing the current session with a restoration of the prior one.

Update: When I wrote this answer, I didn't notice that not all of my Firefox installations had updated to V63. It turns out that the one I tested this on had not yet reached V63. It appears that V63 did away with the background session save and the one-click Restore previous session in History. The recent tabs are listed in History, but you need to reconstruct the session tab by tab. It isn't clear why Mozilla made Firefox less convenient to use in V63.

You can decide whether or not to select "Restore the session" based on how often you want to pick up where you left off, and how many tabs you usually have open. However, a safer strategy would be to set Firefox to restore the session, and just close all of the old tabs when you don't need them. That can be done quickly in a couple of ways:

• Open a new window (Ctrl-N), then close the window with all of the previous session's tabs ("X" in upper right corner).
• Use an add-on to close all tabs. I haven't tried any of these yet, but you can enter "close all tabs" in the add-ons search box, and it shows quite a few options. They vary from simple to ones with a lot of choices, so choose the style that meets your needs.
• If your choice of add-on closes all tabs, which closes Firefox, you can fix that with a setting. In about:config, go to browser.tabs.closeWindowWithLastTab and set it to false. If the last tab is deleted, Firefox will remain open with a new tab.

Update 9/4/19: The previous update was based on the one-click Restore previous session option in History having been removed. It turns out that it wasn't intentionally removed, it was actually a bug or regression. It affected some v63+ releases and not others, and has been discussed in various Firefox bug reports. I just checked this with v68, and it appears to be fixed; the Restore previous session option in History is back. Hopefully, it will stay fixed, in which case, ignore the updates and refer to the original portion of the answer. If that option isn't working in your version, see the first update, above.