Hmm, your Firefox is not behaving normally.

On Windows, Firefox quits when you click the X button on the last window AND if Firefox is set to restore your session automatically, then that single window is restored.

It sounds like your Firefox is automatically restoring your previous session -- otherwise, you would need to select Restore Previous Session first before you find old windows listed under Recently Closed Windows -- but for some reason the window you closed last isn't being treated as an open window; instead it is being treated as a closed window.

Why???

Is there any possibility that another window is closing after the one you think is the last window?

In Firefox this works differently as Firefox can have more than one window open. If you use the close X then you only close this window. If Firefox notices that you closed the last window then the shutdown sequence is started (quit-application). This also means that only the last window you close is restored automatically and you can restore three more windows via "History -> Recently Closed Windows/Tabs". If there are other windows open apart from browser windows then it might even be more complicated and yo7u may have to use "Restore Previous Session".

Best is always to use one of these to close Firefox if you are currently doing that by clicking the close X on the Firefox Title bar, especially if you have multiple windows open to prevent losing tabs in unnoticed windows.

• "3-bar" menu button -> Exit (Power button)
• Windows: File -> Exit
• Mac: Firefox -> Quit Firefox
• Linux: File -> Quit

I verified multiple times that I only had one Firefox window open when testing this. (see note later)

Firefox DOES automatically restore my last session if (and only if) I use ctrl-shift-q or "3-bar" menu -> Exit. If I use the simple X, it does NOT automatically restore my last session (which is the odd behavior).

I do not have multiple Firefox windows open (just the one), even when I first boot and Firefox is Literally the ONLY program running (that isn't windows services), but if I close via the X it almost acts like I opened a new window with a single tab, closed the first window, then closed the 'new tab' window.

.....

-facedesk-

A fellow user of my computer had opened up a session of Firefox in a SECOND DESKTOP. This feature of Windows 11 (and 10) is rather amazing, allowing you to do a myriad of things in visually isolated environments that you can switch between via ctrl-windows-arrow (left and right).

The downside is if you normally just alt-tab, you won't see windows in the other desktop. If you hold the windows key & tab, however, you'll see the others.

Sure enough, there was, indeed, a new tab only window of Firefox open on the second desktop, and this was the 'last' window open when I hit the X.

In the screenshot below, you can see the "bar" across the bottom that shows up when I held the windows key & pressed tab once. Thank you for helping me find my idiocy resolution. :D