Teams. Not my favorite.

The first time you click the "Open Teams" button to use the app for the meeting, Firefox should quiz you on whether it's okay for the site to call an external application to handle the msteams protocol, and then show a second dialog asking you to select which external application to use. That should result in Firefox saving an entry like this on your Settings page:

Has that happened yet?

If not, that is the first problem, we need to get that set up. I notice sometimes nothing happens when I click the button in the page. I usually close that tab, and close the external Teams window and then try the link again. Sometimes it works, sometimes not....

Apparently it's not downloading the file, just trying to use in browser, as according to the attached screenshot, it has not ever seen the msteams protocol.

I've created a script to try to launch the link in the app when it doesn't happen automatically. If you like, you could run this when the meeting page loads (and nothing happens) by pasting this into the Web Console (Ctrl+Shift+K) and either clicking the Run button (if you have the split left-right console layout) or pressing Enter (if you have the classic layout):

// Look for the iframe injected by the Open Teams button
var ifr = document.getElementById('teamsLauncher'); 
if (ifr){ // If iframe is found, load its msteams: address
  location.href = ifr.getAttribute('src');
} else { // Otherwise, look for the "Open Teams" button
  var btn = document.querySelector('button[data-tid="joinInApp"]');
  if (btn){ // Found the button, click it and then after 0.25 seconds load the iframe msteams: address
    btn.click(); 
    window.setTimeout(function(){var ifr=document.getElementById('teamsLauncher');location.href=ifr.getAttribute('src');}, 250);
  } else { // No button, can't help
    alert('Did not find the Open Teams button?');
  }
}

To make it easier to run, I saved a compact version of it as a bookmarklet -- that's a script you run by clicking a bookmark. You can install the bookmark from the following page by dragging the blue OpenTeams button to your Bookmarks Toolbar. Then when the page doesn't work as expected, click the OpenTeams button to try to launch in-app.

https://www.jeffersonscher.com/res/sumomarklets.html#OpenTeams

It works on my test meeting, but I'll be curious to hear whether it works for you.

Sorry for the delayed response, today was the next time that I had an appointment. Your script returns with a popup saying "Did not find the Open Teams button?".

I hope that points you in a useful direction.

Was the Open Teams button visible in the page when you used the link to launch the meeting (see attachment for what I mean)?

I suppose it's possible that the details my script used to find the button might not be universal. That would be a problem...