Possibly there is another "tab group" in that same window. It's not a commonly used feature, and it's going to be removed, but if there are hidden tabs in a window, that's the most likely explanation.

To see all tab groups, you can press Ctrl+Shift+e. You can return to a particular tab by clicking it, or press Esc to exit the tab groups screen.

Thank You for taking your time & answering it.

Perhaps this is the solution. I deleted everything from there. And then opened few new tabs. Now I cannot confirm if that solved my query/issue because it doesn't show any warning while closing Firefox.

Since "Warn me when closing multiple tabs" is hidden in "Options" as it's already set on "True". To confirm that, I checked about:config and its set to "True" Screenshot attached.

(I restarted browser multiple times )

Thank You!

Hmm, okay, I guess you can check for a second tab group if the problem comes back.

The only case I recall of tab groups being created in the background without the user's knowledge involved a video recording program or add-on that was creating hidden tabs and failing to close them. But that probably was fixed because reports of the problem stopped after a while.

I think you pointed it right that I'd the multiple tabs group at the background (which I was unaware of) caused it to show more tabs than actually I was seeing while closing Firefox.

But I think I got into another issue now. I created new tab group with multiple tabs in each but it won't show a warning when I'm closing Firefox.

Thanks for your help. I'll browse forum more. Lets see if I find any solution

If your startup setting changed from home page to restoring previous session windows and tabs, the warning might not be given. I know I never see it with that setting. But I also close Firefox using the menu ("Exit" or the "power" button) rather than using the X button, so maybe that also is a factor.

I tried multiple things but its not working.

But the good thing, I have no unknown tabs in background. And that solves the original purpose of this thread.

Thank You so much for your time! :)