I opened up a scene that I had made using a previous version of Poser in Poser 13 and rendered it. The rendered image looked different from the way that I remembered it looking, so I went and rendered the image in Poser 12, which I still have on my machine. The Poser 12 render looks the way I expected it to. I have made no changes to the scene. I just opened up the scene and rendered it using the Firefly renderer. Can someone help me to figure out why the scene is rendering differently?

Different in what way?

I noticed for Superfly renders, contrast and dynamic range are much better for P13 (even with no PostFX), vs P12 which tends to be a bit flat, but that does mean they don't exactly match. I don't know whether that is also true for Firefly.

I agree with Chromestar. We need more information. Is it lighting, textures, or something else? My tests haven't shown any difference so there must be something different in your image that isn't mine.

It would help if we could see the two images.

I went to create images and now the two programs are rendering the same. I cannot account for what happened. I was not imagining things, and I have the cached renders in both programs to prove it, but for now, things seemed to have fixed themselves. I guess this was one of those issues where the solution is restart the machine and try again.

Every once in a while, I get a render where some object is totally wrong. E.g., their hair is sinking into their head. (For whatever reason, that doesn't seem to happen when rendering animation frames. It's mostly when Poser has been running for a long time and has already done a lot of renders, so maybe a memory issue?) Closing and restarting Poser fixes it. You probably don't have to restart the machine.

You may still have the renders saved. If you select Window > Recent Renders, if they are there you can still export and upload them.
https://www.posersoftware.com/documentation/12/Poser_Reference_Manual/BuildingScenes/Document/Displaying_Recent_Renders.htm