In another discussion in this area, I learned that I can get scrollbars back by going to "Visual Effects" in the Accessibility menu in Windows 11 settings, then selecting "Always show scrollbars." That worked!
I am also having this problem. It does not occur on all web sites or web applications. However, in many applications the vertical scroll bar is extremely narrow, and in some applications I cannot even see a vertical scrollbar in Firefox, meaning that...