firefox scrollbar slider

[Richmond <richmond@criptext.com>]

I have created

.mozilla/firefox/<profile directory>/chrome/userContent.css
.mozilla/firefox/<profile directory>/chrome/userChrome.css
.config/gtk-3.0/gtk.css  /* I think this is the one which made a difference */

and in all of them put:

scrollbar slider {
    /* Size of the slider */
    min-width: 32px;
    min-height: 15px;
    border-radius: 17px;

    /* Padding around the slider */
    border: 2px solid transparent;
}

And set

toolkit.legacyUserProfileCustomizations.stylesheets true

This creates a very large scroll bar on the Profile Manager, but when
the browser starts the usual narrow scroll bar is used. How do I get
wide scroll bars in the browser?

I am using the system theme in firefox.