The new Firefox 100 release is very good, but it has a new feature that inherits scrollbar behaviour from Windows 11. This means you get a floating scrollbar like an Android phone. But this is easily fixed. Search in the Start menu for “scrollbars”. Then click the Always show Scrollbars option. This leads to Accessibility settings and you may then activate this option. This fixes the scroll bar instantly.
I can not work out how to theme the scrollbars with userChrome.css though. It may not be possible anymore.