Oh I think I misunderstood what you are doing, I thought you meant you set the content document body to also have the background-image, in which case the same alignment issue would still happen. But it sounds like you have just one image at the main-window body and the content area is transparent.

With some shuffling you could achieve pretty much the same result visually - so that the header sticks, but doesn’t overlay the list:

@-moz-document url("about:addons"){
  #content{
    max-height: 100vh;
    display: grid;
    grid-template-rows: auto auto 1fr;
    --in-content-page-background: transparent;
  }
  #page-header > .sticky-container{
    position: static !important;
  }
  #main{
    overflow: auto;
    padding-inline: 6px;
    scrollbar-gutter: stable;
  }
}