Maintaining the aspect ratio of divs inside a flexbox seems to not work in Firefox

Question

I'm having a problem with some functionality of flexboxes in Firefox. I'm trying to maintain the aspect ratio of divs inside flexboxes using padding-bottom as a % of the parent and it works absolutely fine in Chrome, Opera, and IE. The sticking point is in Firefox, where the whole thing seems to break down. The test-wrap is't getting a height as it should.

Is this simply an edge-case bug that I should submit to Mozilla or is there a solution to this that I can do with CSS/HTML? We had previously been using resizer images to maintain the aspect ratios but the loading pop-in was unacceptable. I've included an example of the problem below and also made a jsfiddle here: http://jsfiddle.net/sxxbqtnb/2/

.test {
    position: relative;
    height: auto;
    margin: 0 12%;
    background-color: transparent;
}
.test-wrap {
    width: 100%;
    list-style: none;
    padding: 0;
    display: flex;
    justify-content: space-between;
    flex-wrap: wrap;
}
.test-inner {
    position: relative;
    height: 0;
    width: 31%;
    display: inline-block;
    margin: 0 0 150px;
    padding-bottom: 18%;
    background-color: #666;
}

<div class="test">
    <ul class="test-wrap">
        <li class="test-inner"></li>
        <li class="test-inner"></li>
        <li class="test-inner"></li>
    </ul>
</div>

*edited to clean up my css a bit

Answer 1

Try adding display: flex to the upper most parent element, in this case .test

I also gave .test-inner height: 100%. Tested in FF 34.0.5

JSFiddle Link