Rashi on Deuteronomy 33:15/רש"י על דברים ל״ג:ט״ו

1ומראש הררי קדם AND BY THE FIRST THINGS OF THE HILLS OF קדם (taken in the sense of: the hills priority) — the meaning is: and it is blessed through the earliest ripening of the fruits, because its hills are earliest (קדם, "prior") to bring their fruits to maturity. – Another explanation: this tells us that their creation (the creation of the hills in the territory of Joseph) was prior (קדם) to that of all other hills (Sifrei Devarim 353:4).

2גבעות עולם [AND THROUGH THE PRECIOUS THINGS OF] THE EVERLASTING HILLS — i.e., of hills which everlastingly (עולם) produce fruit, and do not cease to do so through restraint of rain.