Customize Super + N Behavior

I’m running Ubuntu 18.04 with GNOME. When I hit Super + 1, the first program in my left favorites bar (in my case, Firefox) starts up. If I hit Super + 1 again, it switches to the existing Firefox instance. I would like it to start a new Firefox process instead.

I followed the instructions in this question, which works if I go into the launcher by hand, but it doesn’t work for the keyboard shortcuts Super + 1, etc. Also, it seems that Super + 1 is not listed as a keyboard shortcut in the settings, so I can’t simply override the binding myself. How can I get a new process to start when I hit Super + N, for N from 1 to 9?