How many lights are you using? Poser 5 is much more sensitive to lights than P4 and three lights in P5 can do the job of 50 in P4. Many light sets available are made for P4 - especially global light sets which WILL shut down P5. What are your Firefly render option settings? The stock 'production' settings allow for settings you may not need - like a 4096 size texture map when you're render max is 3000. P5 allows the user much more control over rendering quality.