I received a response from a submitted ticket, very quickly, actually, kinda impressed, heh. I just took a few days to play around with it.

I *THINK* the below is correct. I didn't 100% understand the answer, but:
   It turns out the "Gamma Correction" setting under the "Firefly" render options will actually change the behavior of the Preview window. If Gamma Correction is "on", the preview renderer appears to use the SueprFly renderer to produce the preview. If the Gamma setting is "off", then the "firefly" renderer is used to produce the preview.

The way that the two renderers produce their "preview" is different.

I tried experimenting more, in general, with node settings and values to see the differences between teh two renderers, but I lost the plot a bit... too many unexpected variables and things I clearly don't understand. Like, the same material chain with the same settings/property values will priduce completely different color resuits in Firefly vs. Superfly,  but this depends on "unexpected" things, like whether the property is animated or not, deviation from default value (rather than just the straight value), etc...

In general, the major difference for the preview is that Gamma setting.