[cromka]

Would be great if it was also possible to have it open the Team URIs in that App Mode instance instead of the browser itself — I assume it does not.

[homebrewer]

What kind of URLs does it use? If it's anything with a unique prefix (like Telegram's tg://), you can add it to the desktop file:

  MimeType=x-scheme-handler/foobar;

and run `update-mime-database`.

If not, I would write a shell wrapper and set it as the default browser; something to the effect of:

  #!/usr/bin/bash
  set -eu
  for arg in "$@"; do
    if [[ $arg == *whatever-url-teams-uses.com* ]]; then
      exec gtk-launch teams "$@"
    fi
  done
  exec gtk-launch firefox "$@"

(gtk-launch uses flags from the .desktop file so you don't have to repeat them)