I haven't been able to test this as my test device is too old. But, I'm betting my money on Firefox somehow rendering `background-attachment: fixed` differently

I haven't been able to test this as my test device is too old. But, I'm betting my money on Firefox somehow rendering `background-attachment: fixed` differently from Webkit/Blink which is causing the slowdown. I'm looking at switching to GPU accelerated transforms instead but that brings a couple of new challenges.