Comment # 15 on bug 1226112 from Frank Krüger

(In reply to Frank Krüger from comment #14)
> (In reply to Wolfgang Rosenauer from comment #13)
> > I will add xdg-desktop-portal as requirement to the next Firefox package.
> > 
> > Another open topic is the default browser check which does not work for me
> > under non-KDE. In addition the "Set as default" in the FF settings seems to
> > do nothing.
> 
> Confirmed within KDE.

Setting widget.use-xdg-desktop-portal.mime-handler=2 in FF seems to fix this.
However, I am unsure about other implications this might have.