> 'Fátt mun at fornu setri fríðs aldjötuns hríðar.'

Ah, maybe it's: "There will be little by way of the ancient
seat/home-of-snowstorm of the beautiful giant" (i.e. there will be few
mountains), because they'll have all fallen down then, as described in
the previous line (Höll taka björg at falla).