I don’t know the current situation especially as regards WebRender in Firefox, but WebRender did box-shadow in a shader back in 2015: https://pcwalton.github.io/2015/12/21/drawing-css-box-shadow....

I don’t really know what I’m talking about with all this stuff in much depth; I’m mostly reporting what I’ve heard and learned over the years of watching from the outside and keeping half an eye on things, but never quite learning about them in detail.

Since we’re talking box shadows and performance, https://raphlinus.github.io/graphics/2020/04/21/blurred-roun... is also some fun work in the space.