Thanks for your reply bepeg4d!
The scene is lit with the Daylight environment, no custom HDRI is present. As mentioned, the fireflies are only apparent in the Sunlight pass, the main render looks absolutely fine. Clay rendering doesn't look much different since there is glass on the exterior windows which blocks all sunlight if Clay mode is enabled...
Now that I think of it, it's probably worth testing to hide the window glass from the scene since there's really no visible reflection on the glass surface, and having glass on the windows only serves to slow down the rendering. It's a likely culprit for the fireflies, since the sunlight is pouring in through the glass.
For testing, I disabled the interior lights which makes the fireflies more apparent in the Beauty render as well. Likely the artifacts are just less visible when the composited render adds the contribution of the other light sources on top of the sunlight.
I'll update this thread after removing the glass from the windows, to see if that gets rid of the fireflies!