[CentOS] How to add an application icon?

>
> > How to add an application icon to gnome 3 classic desktop on centos 8?
> > TIA
>
> Using firefox as an example,
>
> cp /usr/share/applications/firefox.desktop $(xdg-user-dir DESKTOP)/
>
> or
>
> ln -s /usr/share/applications/firefox.desktop $(xdg-user-dir DESKTOP)/
>

I've already tried something like this but the icon is that of a text
document and it just opens as text when clicked. Do I have to install a
shell extension for this to work?