Aaaah, publication politics.
Unfortunately this happens quite frequently, and in some groups the authorship practices make no sense at all. I've been fortunate that my PIs have all been fair, but I know a few people who have been stung by political authorship decisions. It can be a very emotive issue, try to keep level-headed and polite. You can also talk to the student currently in the first author slot and see if she thinks it's fair. I know of situations where the first and second authors of a manuscript have approached the PI together to request an order change (and bizarrely were refused by the PI).
My advice is:
1. If you disagree with the PI's chosen order, ask him/her to explain the contributions of the author you think has been inappropriately placed, and prepare a summary of your own contributions for comparison. Don't be aggressive - remember that the author you feel is misplaced may have done more than you are aware of (often the case in interdisciplinary work), so be prepared to back down gracefully if that is the case.
2. If you still disagree, objectively and politely make your case. You can also point out that you will soon be looking for work, and that it is important to you that the authorship order is fair. A lot of seasoned PIs seem to have forgotten how important authorship order is to young researchers and don't realise the grief that it can cause.
3. If you and your PI can't agree, you can suggest the compromise of joint-first authorship (the "equal contribution" footnote). This is a good tactic when confronted by a dictatorial PI who refuses to justify or change the order they have chosen. Remember that your PI has the final say - there is no sense in causing a big argument, especially if you have more papers in preparation with them. A dictatorial PI may punish your insubordination with more unfair authorship placement in future manuscripts. While you may not feel that joint-first is necessarily a fair reflection of author contributions, it might be the best option you have.
4. If you have a lot of (potentially) first-author publications in the pipeline, you should consider whether this is a fight worth fighting - especially if your PI is the dictatorial type. You may have to make the political decision to let it go and maintain a good working relationship with the boss (from whom you will need references soon).
As you will have guessed by now, I tend to avoid confrontation. Others may prefer more aggressive approaches. It depends what kind of person you are and the mentality of your PI.
Good luck, I hope it works out.