I don't think you can be first than someone, but I was thinking like some sort of competition.
Would this be considered: The one that done it before you do?
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First is one of the few adjectives in English that does not have a comparative. It's a true absolute adjective: either something is first or it's not. You can't say I did it firster or I did it more first.
While many things that grammarians claim are absolute adjectives, like perfect, complete, and true, are really not, first really is one. People say more perfect, more complete, and truer all the time, but nobody says firster or more first.
And when you say "adj. than you", you need a comparative. So first than you, and firster than you are both wrong.
This Ngram is proof that first is a true absolute adjective, unlike perfect, complete, and true. Note that these few instances of more first that it finds are almost all expressions like more first prizes.