Where are you seeing this. You need to provide screenshot and how your verifying this O/S notification?

I don't think that the Firefox user agent is updated to show Windows 11 as I still see "Windows NT 10.0; Win64; x64" in other questions that show the OS as Windows 11, but in that case the Tags show Windows 11. So if your question is about this then can ignore it.

Just to be sure, you can check that you do not run Firefox in compatibility mode for Windows 10. You can open the Properties of the Firefox desktop shortcut via the right-click context menu and check the "Compatibility" tab. Make sure that all items are deselected in the "Compatibility" tab of the Properties window.

Locking this thread.
Please continue here: [/questions/1402259]