I have div that contains flexbox element with iframe inside (made to be responsive using CSS).
In Firefox that parent's height is 0 for some reason.
.container {
border: 2px black solid;
}
.flexbox {
max-width: 854px;
margin: 0 auto;
display: flex;
justify-content: center;
align-content: center;
align-items: center;
background: yellow
}
.flexbox div {
position: relative;
width: 100%;
padding-bottom: 56.25%;
}
flexbox div span {
position: absolute;
top: 0;
bottom: 0;
left: 0;
right: 0;
width: 100%;
height: 100%;
border: none;
}
<div class="container">
<div class="flexbox">
<div>
<span>
TExT
</span></div>
</div>
</div>
The best way to see this please look at my example on JSFiddle: https://jsfiddle.net/5jLqugy6/