My first authorship is being turned into co-first authorship, what can I do?
First, try to calm down and don't react hastily.

Your situation is very frustrating, but it is not horrible. Let's say that the current author ordering would be the final one. You would be what's known as a co-first author. While it would be slightly more advantageous to be the sole first author, in my experience being co-first practically will not have a significant negative effect on your CV (BTW - are you a grad student or postdoc?). This is important for you to realize - the worst-case scenario that you mention is upsetting, but definitely not a disaster.

Some additional points:


You can try explaining your perspective to your professor. If you originally decided that you would be the sole first author, you should definitely remind him/her of that. Do this politely, of course, but show your professor this is a big deal for you. This could cause the professor to reconsider, depending on his/her personality.
Instead of being the sole first author, you can consider being co-first but listed first. This doesn't make a big difference, but people might associate you with the paper more easily (Whatshername et al.).
Consider that regardless of the number of figures, the other author might deserve being co-first. For example, in many papers an experimentalist produces the data and a computational person analyzes the data and produces most of the figures - and they would often be equal co-first authors (even though each of them could think of themselves doing "most of the work"). So I do not know all the details of who did what, but it does not sound completely implausible that the other person did work which justifies being co-first.
Regardless of what happens, I suggest you talk with your professor about how to get a more satisfactory outcome next time (if this is relevant for you). Discuss authorship in advance and what happens when things change.


Good luck!