There is an attribute iray_nominal_luminance which directly relates to Firefly Filter. Firefly Filter is enabled by default according to the scene options in .mi export.
Here is what NVIDIA says: =========== The nominal luminance is a hint to Iray Photoreal on what is considered a “reasonable” luminance level when viewing the scene. This luminance level is used internally to tune the firefly filter and error estimate. When the nominal luminance value is set to 0, Iray Photoreal will estimate the nominal luminance value from the tonemapper settings. If a user application applies its own tonemapping without using the built-in tonemappers, it is strongly advised to provide a nominal luminance. ===========
To compare the influence of the iray_nominal_luminance, I enclosed 2 screenshots of identical scene renders.
First slide is with default scene options and second with "attribute scalar "iray_nominal_luminance" 200"
200 was the value I selected experimentally while working with the same scene in DAZ
As you may see on the second slide fireflies are suppressed more aggressively. So it is important to have this control available within the control panel so that we can choose the best possible level of fireflies suppression.
Side Note: AI denoiser may of course suppress fireflies even further, but with low number of iterations and under certain lighting conditions, it may not be as effective dealing with excessive amount of fireflies. Denoiser would smudge them creating artificial stains, which then would flickering in animation.