First issue, dbzer0 has this background that Firefox scrolls past as you scroll down. Every other browser lets the background stay static. Firefox does not.

Seems like this is a bug with Firefox on mobile.

Works as expected on desktop, even when in Responsive Design Mode (Ctrl+Shift+M) and using a phone form factor (landscape and portrait seemingly doesn't matter).

Not sure what's going on on the mobile version of Firefox there. Bit odd. Maybe there's a bug report somewhere.

I found that it does scroll past the background, but there's something that happens some ways down the page that makes the background get covered from the bottom and up.

This is evident when scrolling down in landscape mode.