Slack Flatpak not compatible with Firefox Flatpak

Links in the Slack Flatpak in Debian Linux (11) want to start a browser installed by dpkg (apt) and not the Flatpak.

I tried this:
xdg-settings set default-web-browser org.mozilla.firefox.desktop

And Libre Office I noticed uses the Firefox Flatpak to launch a link.

How can I get the Slack Flatpak to launch a link in Firefox Flatpak?

With no browser installed via dpkg/apt I see a picture of a handbag and this message:

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