App opens two Firefox browser windows

I’m a long time Safari user. I’m interested in switching to Firefox. My app has a line that opens up my browser and displays the given website (“textweb.text”)

ShowUrl textweb.text

But when I switched my default browser to Firefox, it now opens two Firefox windows, one with the Firefox homepage and he the other with my intended web page (“textweb.text”)

Sorry in advance if this is more of a browser question, but I cant seem to figure it out in Firefox Preferences.
on MAC

It’s an issue with Firefox and only happens if Firefox is closed when you ShowURL. If Firefox is open, it should either open a new window or tab. It’s a quirk, but I still prefer Firefox over Safari.

When I was playing around I did notice if Firefox was already open, it didn’t open a second window…