I'm assuming that you mean is it possible for someone to 100% complete this game before they defeat Majora, and not just on their first save file. Assuming this, it isn't impossible, as you don't need to have beat Majora to acquire anything as far as I'm aware. Sure, it takes a lot of skill and time, especially since a lot of players won't know when they need to brack track to certain areas after certain points in the game to collect items. The odds of this happening are extemely low, as I said above, it would take a lot of skill. But yeah, it's not impossible. However, correct me if I'm wrong with what I'm assuming. If you just meant on their first playthrough, without a guide, then yes, it's possible on all games.
The only games that are impossible to beat on your first file before you beat the final boss are those games in which you can only acquire certain items after you beat the final boss. It's usually annoying when games are like this though, as you won't really have anything to do with the new item because you have completed the entire plot.