I am on Windows 11 and needed to debug a problem with a scrollbar today, the bug only appeared with the old scrollbar style, so I needed to enter about:config, change widget.non-native-theme.scrollbar.style to 4 and restart the browser.
Not that great.. I would really like to have this as a simple switch in the devtools.
Or add a regular setting like mentioned here: Scroll bar width and customization options