5 Save the gtk.css file and log out and back in to Gnome.
6 When you log back in, you should see the scrollbars on apps like the Firefox browser and terminals changed to the value you entered in the gtk.css file.
You may have to tinker with the min-width to get the scrollbar thickness you want but hopefully this resolves your problem. Thanks to the folks on this ubuntuforums thread for the answer.
My environment is CentOS 7 and Gnome Shell 3.25.4, but this problem seems to be more generally related to Gnome 3.
If you want to know your Gnome Shell version, the easiest way to find it is to open a terminal and type gnome-shell –version.
If this doesn’t resolve your issue, I’d like to hear about your situation. I understand if registering to comment is too much work though … 🙂
If you’re having slow and jerky scrolling in Firefox with Gnome 3, this is worth a try, especially if your Chrome scrolling is fine.
1 Open firefox.
2 Type about:config in the Firefox address bar and hit Enter.
3 Type mousewheel.min_line_scroll_amount into the search bar.
4 Double click the row that shows up, enter a higher number (I use 30) and press Enter.
You may have to tinker with the mousewheel.min_line_scroll_amount to get the behavior you want but hopefully this resolves your problem. Thanks to the folks on this reddit threadfor the answer.
My environment is CentOS 7 and Firefox 60, but this problem seems to be generally related to recent (e.g. ~2018) Firefox on recent Gnome.