Thanks for reminding me, Learn.Microsoft.Com, that I needed to do this on my workstation's #Firefox as well:

1. Go to about:config

2. Search for the setting: layout.css.scrollbar-width-thin.disabled

2.a. Just searching for "thin" should be enough to find the setting.

3. Toggle from "false" (its default) to "true".

Reload the page whose front end designers made the scrollbars thin, to get proper normal scrollbars on it.