I contacted the technical department at e-frontier.com and have just received this reply which is a superb example of excellent customer service and it's far more detailed than I would have expected. I present it here in full in the hope that it will assist other users with a similar problem.
"Dear Mr. Hayes,
Hmmm. It sounds as though you're more than a little familiar with Poser and the general workflow; my normal advice in a case like this would be to advise you to start with the most basic Firefly settings- fairly small render size, perhaps 1024x768 or thereabouts, maximum texture size set to perhaps 1000-2000 pixels (since you're rendering to an image no larger than 1024 pixels, the image maps will get scaled down on the render anyway, so you don't lose any quality in the final render by telling Firefly to subsample them when it loads.) I'd suggest turning the minimum shading rate up to 4 or so, and leave raytracing off, and see how that goes- obviously that won't give you the image quality you want but it should serve as a starting point to see which settings cause the most problems. From there you'd probably want to start by decreasing the minimum shading rate to 1.0 or so, then turn up the render dimensions, then (very carefully) adjust the max texture size.

Given that you're an old Poser hand, I can skip the usual advice about not using too many shadow-casting lights and keeping your scenes simple; I imagine you've also tried deleting your Poser preference files, just in case they've gotten corrupted (although your description of the problem doesn't indicate bad prefs.) My guess in this case is that either (a) Firefly is choking on too many large image maps (this is unfortunately rather common, and we're working to improve its ability to handle image maps) or (b) that there's some other problem that stems from either a faulty Poser installation or something about the scene- complex Material Room setup?- that runs Firefly out of memory. Given that you have as much RAM as Poser will address, it might be worth adjusting your Adaptation Threshold settings in the Misc. tab of the General Preferences- changing the Memory Limit Buffer would give you at best an additional 32Mb of RAM to work with, which probably wouldn't be enough to get the job done and would expose you to crashing rather than simply terminating the render. Adjusting the adaptation threshold, though, might get the job done as raising that value causes Firefly to reduce the bucket size quicker, rather than trying to hang onto bigger bucket sizes; this makes for a slower render but more efficient memory use.

By way of troubleshooting, I'll suggest the following-

1. Try setting up your render with the minimum shading rate set to 2 and the maximum texture size set to 1000 pixels or so. If you're rendering a 3000x3000 pixel image with the foreground figure taking up around 2500-2750 of that, her face won't take more than 500 or so pixels, so putting a 4000-pixel image map on it is overkill- if you were doing a closeup on her face it'd be a different story. Turn up the adaptation threshold settings to 0.75 if you can't get a good render with these settings using the default value of 0.375.

2. If you can render successfully with these settings, try turning the minimum shading rate down to 1.0. You can turn on raytracing as well if you've got reflection or refraction (water, glass, etc.) in the scene, otherwise leave it off.

3. Finally, if you're successful with the min shading rate set to 1.0, you can start increasing the max texture size, although again you should use a setting that reflects how big (in pixels) the biggest object for which you want to see high levels of detail is going to be (does that make sense?) You can also try turning the minimum shading rate down to 0.5 but going below 1.0 increases the render time and resource requirements a lot for diminishing returns- unless you're using displacement maps or other complex Material Room functions.

I hope this helps!
Regards, Colin Gerbode
Technical Support"

I hope you find this of interest.
Tony