When we close the Mozilla Firefox window with multiple tabs, Firefox asks to confirm the action. This prevents you from accidentally closing the whole window when you intended to only close the current tab. So many times it really helped me.
There is also an option for un-checking Warn me when I attempt to close multiple tabs. This is to disable the "Close Multiple Tabs" warning from next time and allow Firefox to automatically close the window with all tabs.
You might have found this a weary feature and disabled the warning. So, the box never appears again. Now how will you restore this warning again? Is this as simple as to disabling this?
The answer is yes! it is very simple. Follow this steps.
Step 1: Click on orange Firefox button at the left top most corner and navigate to Options menu.
Step 2: In the Firefox Options window, go to the "Tabs" tab and check the "Warn me when closing multiple tabs" option. This is at the second position in the list.
Step 3: Now hit the OK button at the bottom of the window. Your problem has been resolved now. Be ready to endure the confirmation box again when you close the window. Ha Ha.