How to fix height: auto with flexbox in Firefox?

.portfolio {
  width: 100%;
  position: relative;
  overflow: visible;
  display: flex;
  flex-direction: row;
  flex-wrap: wrap;
  justify-content: space-between;
  background-color: red;
}

.portfolio a {
  display: block;
  background-color: black;
  width: 30%;
  height: auto;
  padding-top: 30%;
  /* 1:1 Aspect Ratio */
  position: relative;
  /* If you want text inside of it */
  margin-bottom: 5%;
}

<div class="portfolio">
  <a href="#">
    <div class="portfolio_item">
    </div>
  </a>
  <a href="#">
    <div class="portfolio_item">
    </div>
  </a>
  <a href="#">
    <div class="portfolio_item">
    </div>
  </a>
  <a href="#">
    <div class="portfolio_item">
    </div>
  </a>
  <a href="#">
    <div class="portfolio_item">
    </div>
  </a>
  <a href="#">
    <div class="portfolio_item">
    </div>
  </a>
</div>