Starting with Firefox version 94, you no longer get a warning when you attempt to close a Firefox window with multiple tabs open. Luckily, the feature is not fully removed, and you can enable it from Firefox’s settings menu.
Get a Warning When You Close Multiple Tabs in Firefox
To re-enable the missing prompt in Firefox, first, open Firefox on your Windows, Mac, Linux, or Chromebook computer.
In Firefox, at the top-right corner, click the three horizontal lines.
From the menu that opens, select “Settings.”
On the “Settings” window, in the left sidebar, click “General.”
On the “General” screen, in the “Tabs” section, enable the “Confirm Before Closing Multiple Tabs” option.
Tip: To again disable the prompt that appears when closing multiple tabs, turn off the “Confirm Before Closing Multiple Tabs” option.
And that’s all there is to it. Going forward, when you close a window with multiple tabs open in it, Firefox will display a warning before the window is actually closed.
Did you know you can selectively close multiple tabs at once in both Firefox and Chrome?