And yeah, of course a grid of images is going to be fast, especially if they aren't even scaled. It's literally just a blit. What gets expensive is when you add bilinear filtering, alpha compositing, text, shadows with blur, path filling and/or tessellation...
All of those things are things people now expect from apps, native or otherwise.
There wasn't any fancy shadows, compositing, etc. The CPU was all spent calculating layouts and doing "stuff", before the images even rendered. My guess is some suboptimal code buried by a kilometre's worth of abstractions. It's not that FF is doing something slow (that I know of), it's the mess people have built on top of HTML/JS/CSS that ends up with non-junior developers creating monstrosities.
And yeah, of course a grid of images is going to be fast, especially if they aren't even scaled. It's literally just a blit. What gets expensive is when you add bilinear filtering, alpha compositing, text, shadows with blur, path filling and/or tessellation...
All of those things are things people now expect from apps, native or otherwise.