I have an example where I use background-attachement:fixed; and filter:blur(15px); on the same element. Currently all browser I tested does the same. Applies blur only to the visible area of the fixed image, which I think is bad as filter should apply to the whole element. You can see the result on this example: https://smartslider3.com/bugs/firefox/blurfixed/test.html

When you scroll down, you should see that a "box-shadow" like effect follows the visible rectangle of the image. I think that effect should stay on the whole div and when you scroll down, it should be visible at the top and at the bottom of the real image only.