There’s an old bug report about Firefox playback being limited to 44.1Hz or 48Hz:
This makes me think it might be related to the playback sample frequency, e.g. Firefox is playing at 44.1Hz but other digital outputs are being played back at an incorrect higher rate? (This could also explain the slightly WIndows 10+Bluetooth sound if playing back at 48Hz instead of 44.1Hz.)
Have you tried setting default-sample-rate to your card’s native output rate, or setting avoid-resampling = true ?
Another idea would be to check some other Pulseaudio settings e.g. based on suggestions in this thead: