On Windows, Firefox follows the system setting (System Settings > Accessibility > Visual Effects > Always show scrollbars)

Or, you can try an extension, like - Custom Scrollbars https://addons.mozilla.org/en-GB/firefox/addon/custom-scrollbars/

If the Windows settings don't achieve what you want, there may be preferences in about:config which do. Search in about:config for widget.non-native. You may have the following which could help: widget.non-native-theme.scrollbar.style You can use the following values: 0: Default platform scrollbar style. 1: macOS scrollbars 2: GTK scrollbars 3: Android scrollbars 4: Windows 10 scrollbars 5: Windows 11 scrollbars. I understand that Windows 10 scrollbars are wider than those on Windows 11 (I don't have Windows).