The Flexbox properties you're using come from the old 2009 draft and should never be used without also providing the standard properties. On top of being buggy, broken, and incomplete (particularly the Mozilla implementation), they will eventually be dropped in favor of the standard properties.
http://codepen.io/cimmanon/pen/iCfEu
#wrapper {
width: 100%;
margin: 0 auto;
background: #333;
}
#container {
display: -webkit-box;
display: -moz-box;
display: -ms-flexbox;
display: -webkit-flex;
display: flex;
-webkit-box-pack: justify;
-moz-box-pack: justify;
-ms-flex-pack: justify;
-webkit-justify-content: space-between;
justify-content: space-between;
width: 80%;
margin: 0 auto;
background-color: black;
}
#container div {
background: #0f0f0f;
text-shadow: 0px 2px 0px rgba(0, 0, 0, 0.3);
-webkit-box-flex: 1;
-moz-box-flex: 1;
-webkit-flex: 1;
-ms-flex: 1;
flex: 1;
overflow: hidden;
text-align: center;
border-radius: 5px;
/*Shadow*/
-webkit-box-shadow: 0px 6px 0px rgba(0, 0, 0, 0.8);
-moz-box-shadow: 0px 6px 0px rgba(0, 0, 0, 0.8);
box-shadow: 0px 6px 0px rgba(0, 0, 0, 0.8);
/*Gradient*/
background-image: -webkit-linear-gradient(top, rgba(0, 0, 0, 0), rgba(0, 0, 0, 0.2));
background-image: -moz-linear-gradient(top, rgba(0, 0, 0, 0), rgba(0, 0, 0, 0.2));
background-image: -o-linear-gradient(top, rgba(0, 0, 0, 0), rgba(0, 0, 0, 0.2));
background-image: -ms-linear-gradient(top, rgba(0, 0, 0, 0), rgba(0, 0, 0, 0.2));
background-image: linear-gradient(top, rgba(0, 0, 0, 0), rgba(0, 0, 0, 0.2));
padding: 10px 0px 10px 0px;
width: 75px;
margin: 10px 15px 10px 15px;
}
.hide {
display: none;
}
Note that I've eliminated all of your unique flex properties for your flex items, using justify-content instead. It should also be noted that using the table/table-cell display properties instead of Flexbox would be by far more efficient and have better browser support.
#container {
display: table;
width:80%;
margin:0 auto;
background-color:black;
}
#container div {
background: rgba(15,15,15,1);
text-shadow: 0px 2px 0px rgba(0, 0, 0, 0.3);
overflow: hidden;
display: table-cell;
text-align: center;
border-radius: 5px;
}