[NOTE: I am told that there is now a Firefox Add-On called "Always Ask" that may fix this, so click the link to download it, and give it a try before you go through all the steps on this page. Then, if that doesn't work for you, come back here and try this.]
THE PROBLEM: Many users notice that Firefox no longer prompts or warns the user before closing or exiting Firefox windows with multiple tabs opened in the window. This warning when closing multiple tabs not working ‘error’ happens despite the fact that the “Warn me when closing multiple tabs” option in Tabs section of Firefox Preferences has been checked and enabled!
With the “Warn me when closing multiple tabs” option selected, Firefox is supposed to pop up a dialog box titled “Confirm Close” asking “You’re about to close X open tabs. Are you sure want to continue?” as happened in earlier versions of Firefox, however, this is no longer the case, and Firefox may now just close and quit without warning or confirmation even with more than one tab open, and, even worse, this happens despite the fact that the user has set their preferences otherwise!!
THE SOLUTION: To be alerted every time you close a Firefox window with multiple tabs, even when quitting:
First, open up the "Firefox>Preferences" (Mac) or "Tools>Options" (PC) dialog window.
On the "Main" tab, set your "When Firefox Starts:" preference to: "Show my homepage" or "Show a blank page"
Then, on the "Tabs" tab, click the little box to set your "Warn Me When Closing Multiple Tabs" preference to "on," and close the Preferences/Options dialog window.
(You've probably already done all that and it hasn't worked, which is why you're here, but I have to include it, just in case.)
Now comes the good part: Type about:config into the browser window's address bar, press Return and then click "I'll Be Careful..." to get into Firefox's Hidden Preference Settings. Oooh, scary!! (Not!)
Scroll down to find browser.warnOnClose and if it's not already set to "true," double-click it to set it to "true."
Now find browser.tabs.warnOnClose and if it's not already set to "true," double-click it to set it to "true."
Then find browser.tabs.warnOnQuit and if it's not already set to "true," double-click it to set it to "true."
NOTE: If there is no browser.tabs.warnOnQuit item (and there probably won't be), you will have to make your own, but don't worry, it's easy-peasy, mate!
Here's how to do it:
Control-Click (Mac) or Right-Click (PC) anywhere on the page and select New>Boolean from the pop-up menu that appears.
Enter browser.tabs.warnOnQuit as the Preference Name (note the capital O and Q) and click OK
Select true in the next window and click OK
That's it, you now have a browser.tabs.warnOnQuit setting, and it's set to true, so close the about:config tab.
Now, the next time you press Command-Q (Mac), or click the Red X (PC), whether accidentally or on purpose, Firefox will ask you what you really want to do BEFORE it quits with a "Warning" dialog box where you can click "Cancel." Hooray!
(Bear in mind, however, that this only works with multiple-tab-windows... if you have a browser window open with no additional tabs open in it, and you accidentally quit, it's just gonna quit, my friend, with no warning at all!)
And, finally, if you ever want it to stop doing this, just click "Do Not Ask Next Time" in the Warning box, and it will go back to the way it was originally. Then, of course, if you want it to start warning you again, just repeat the steps above to turn it all back on!
Good luck, and, if this works for you, please spread it around! Link to this page, tell your friends, call the neighbors, wake the kids, but please do let other folks know how to fix this really dumb Firefox problem!!
Or, if, for some reason, this doesn't work for you, well, sorry... this is what worked for me on my Mac, but, as always, "your mileage may vary," as they say.
BTW, since I'm on a Mac, I had to guess at some of the Windows steps, so, if I got any of it wrong, sorry, just let me know in the "Comments" section, below!