AFAIK it's a known issue that a HDRi in the reflection override slot can cause (a lot) fireflies. You can solve this by clamping the reflection HDRi, but this gives somewhat more dull reflections.
I believe this is a bug, but untill it's solved, we're stuck with either dealing with fireflies or dull reflections. So I n cases where you don't absolutely need a different reflection HDRi than the one used for lighting, you should avoid using the reflection override.