Firefox is a little different when it comes to absolute positioning. Try to understand what actually absolute position works. The element having abs with height: 100% will cover fully if the parent container is also 100%

you also have a padding-bottom i don't understand why do you need padding bottom, this will make your children appear smaller.

Try adding width: 100%; to img-container. i tinkered with your code in the inspector. it seemed to fix it. Try and let me know how it goes