In the Windows 11 search box (on the Start menu or Taskbar) type scroll bar and look for something similar to "Automatically hide scroll bars in Windows". Click that to open a control panel so you can switch off that setting. Does that work on yours?

Alternately, there might be a way to set Firefox to ignore the system setting (if you like the invisible/hover scroll bar on other applications), but I'm still on Windows 10 so I haven't experimented.

See also the scroll bar setting in Firefox.

• Settings -> General -> Browsing -> Always show scrollbars