I'm using XFCE on 18.04 but this seems to be applicable to all desktop environments. Most (bigger) applications (try for example with Thunderbird) behave like this:
If there is already an application window, this window is shown. No new windows are opened. This works if the window is on another workspace, too.
Firefox 66.0.3 opens a new window any time I click the starter icon in my XFCE panel. Chromium behaves like Firefox. This is a typical Linux issue. On Windows, I can't remember to have more than one browser window, except I explicitly told them to open a new window.
This is quite annoying because at the end of the day I have many lost Firefox Windows open on my workspaces.
Is there a way to tell the starter icon to open a new tab in an existing window instead? Or just showing the existing window?