Does the suggestion in the article not work? There are few options in about:config for scrollbars so I'd look at the others too. If that doesn't work I'd bet https://old.reddit.com/r/FirefoxCSS/ has a solution.
That is confusing... I'm not sure what the unit of measurement there even is. Something like pixels would be nice, but that can't be it with a default of 1!
about:config is invaluable, but better documentation on the options would really be nice.
I changed the setting for widget.non-native-theme.scrollbar.size.override from 1 to 50 and it increased size.
The initial value of 1 was confusing me because I kept thinking it is Boolean 1.
Posting a solution here in case someone else finds it useful.