That is an option you can set.

Type about:config<enter> in the address bar. If a warning screen comes up, press the I Accept the Risk button. At the top of the screen is a search bar.

Set browser.tabs.warnOnClose to true

Thank you. Sorry. I should have said so. It's already set to true.

On verifying that again, to make sure something had set it back to false, I did a search for "warnon", and I found another setting:

       browser.warnOnQuit

Setting that to true seems to have done the trick.

It makes me wonder what browser.tabs.warnOnClose does.

Oops, sorry about that. Good work.

I'm having this exact same problem (Win 7 FF 57 32-bit) but both of the above settings are set to true and it's still happening. I've also tried it with a clean profile, in case an extension was causing the problem, but no dice.

Are you saying the browser is closing on its own, or did it crash? What was in the tabs when it closed?

The same webpages? Different pages? How long before there is trouble?

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

Thanks for the reply, FredMcD.

No, I'm saying when I click on the close button [x] it doesn't warn me when I've got multiple tabs open. It's the same in safe mode.

Okay, it gets weirder…

If I launch a new window then go to close the original window (with multiple tabs open) it warns me. If I open several tabs in a new window and try to close either window, it warns me. But as soon as only one window is left open, it doesn't warn me. Private windows don't warn me, regardless of how many windows are open. Whereas in all cases, right-clicking on a tab and selecting 'Close other tabs' always warns me.

The thing is I'm pretty sure this wasn't happening when I first updated to Quantum a couple of days ago. Since then I have added a couple of new extension – uMatrix and Violentmonkey – but as I said before, I've tried this in a new clean profile and it's just the same.

Another thing I've done since updating to Quantum is try out the Pocket extension. I logged in using my existing Firefox account and saved a couple of pages. But I've subsequently logged out and even tried disabling pockets altogether in config, but it's made no difference.

The only other thing I can think of is I used to have Tab Mix Plus installed, but it was legacy and not working any more so I removed it. But I've noticed it's left behind a bunch of settings in config, could they be having an effect?

Antagony At this point, you should ask your own question.

Please go to the top of any support.mozilla.org web page and use the Ask A Question link; https://support.mozilla.org/en-US/questions/new Select the product for what the problem is (Firefox or whatever).

Now select the category for the problem. At the bottom is Other.

Describe the problem in brief. Then press <enter>. The website will display any prior posts that might help you. If nothing on the list can help, at the bottom press the button; None Of These Solve My Problem.

Now enter all the relative information on the problem. At the bottom of this, you can Add Images.

Under this is Troubleshooting Information. Please press the Share Data button. This will let us look at your system details. No Personal Information Is Collected.

Thanks FredMcD, I will do that in my break later today.