i've been raised as a Gen Z to learn "first nations", though aboriginal (from the root word aborigine) also means the exact same thing, so i personally don't comprehend how someone can find offense in using that word.
maybe they are used to seeing aboriginals to describe aussi natives? still, it essentially means "first of the region", or in other words, "first of the nation".
indigenous, aboriginal, and aborigine, mean exactly the same thing. anyone getting offended at any of these word usages probably doesn't know the definiton.