How can I make other apps display the same, slender kind of scrollbar that for example Files (the default file browser for Ubuntu) uses?
To be precise I want this scrollbar everywhere, especially in Firefox
rather than this one vaguely reminiscent of Windows 98
In Ubuntu 16.04 support for the overlay/Unity scrollbars was dropped in favor of the Gnome scrollbars. The relevant bugs concerning the old scrollbars have been closed as won't fix.
Since Firefox didn't use GTK3, however, even those didn't work when 16.04 was released. Firefox 46, however, finally includes GTK3 integration, so the Gnome scrollbars are now displayed.
As for other apps, it depends on their support for GTK3. LibreOffice, for instance, has some support (just download the
libreoffice-gtk3
package to see the scrollbars), but it currently looks very broken with any dark theme (e.g. Ubuntu's default Ambiance).