Hi @Kent Matheson  

This is actually a frequently-asked-question, although here by the word “frequently” I mean “twice,” your post being the second I’ve seen. AFAIK it is not a Miro setting but a browser setting. 

I asked the Thai wise man Chat Gee-Pee-Tee and he recommended what I included below.

Hope this helps! Cheers, Ken

• Open Firefox Preferences:

  • Click on the three horizontal lines (menu button) in the upper right corner of the Firefox window.
  • Select "Settings" from the dropdown menu.

• Navigate to Applications Settings:

  • In the Settings tab, select the "General" panel.
  • Scroll down to the "Applications" section.

• Find Miro Links:

  • In the search bar within the Applications section, type "Miro" to filter the list.
  • Locate the file type or protocol associated with Miro links (it might be listed as "miro://" or something similar).

• Change the Action:

  • Click on the drop-down menu next to the Miro link type.
  • Select "Use Firefox" from the list of options.

• Save and Close:

  • After making this change, close the Settings tab.

Hey, thanks very much.
I’m using chrome though generally not firefox, though may switch if I can get this setting in place… 
I can’t find the setting option you’ve specified in the FF Settings though, under the Applications section there’s only options related to downloads not page opening behaviors.

It seems the wise man is maybe a little out of touch… though you have given me hope, and I’ll keep digging. And apparently I owe an apology to Miro...