Firefox consults your system-level setting for overlay-style scroll bars. On Windows 11, past threads say:

Go to Accessibility settings > Visual effects and enable "Always show scroll bars"

Does that fix it?

If the Windows setting doesn't fix the issue, the Firefox preference widget.non-native-theme.scrollbar.style has the values listed below. You may also have a preference widget.non-native-theme.scrollbar.size.override After opening about:config search for widget.non 0: Default platform scrollbar style. 1: macOS scrollbars 2: GTK scrollbars 3: Android scrollbars 4: Windows 10 scrollbars 5: Windows 11 scrollbars

https://support.mozilla.org/en-US/kb/about-config-editor-firefox