Firefox UI line "glitch" - html

Question

I have a "small" issue on my website with Firefox. On Google Chrome and Safari it is working just fine.
What it should be (Chrome and Safari):
Issue on Firefox:
.headline {
line-height: 1.5em;
position: relative;
}
.headline:after {
content: ' ';
position: absolute;
display: block;
width: 230px;
margin: 5px 2%;
border: 2px solid #64c800;
border-radius: 4px;
-webkit-border-radius: 4px;
-moz-border-radius: 4px;
left: 50%;
transform: translateX(-50%);
}
<h2><span class="headline">Contact</span></h2>
The expected result is that the green line should be below the header (in this case below "Contact".
Reproducible code:
Important forgotten note; I use Bootstrap 4 too.
<style>
#wrapper {
position: relative;
margin: 25px auto 5px auto;
max-width: 820px;
min-height: 600px;
padding: 1px 0px 30px 0px;
background-color: #ffffff;
color: #603813;
text-align: center;
}
.headline {
line-height: 1.5em;
position: relative;
}
.headline:after {
content: ' ';
position: absolute;
display: block;
width: 230px;
margin: 5px 2%;
border: 2px solid #64c800;
border-radius: 4px;
-webkit-border-radius: 4px;
-moz-border-radius: 4px;
left: 50%;
transform: translateX(-50%);
}
</style>
<div class="container-fluid">
<div class="row-fluid">
<div id="wrapper">
<div class="row">
<div class="col-md-12">
<h2><span class="headline">Contact</span></h2>
</div>
</div>
</div>
</div>

Answer 1

I think the problem is that you don't define a vertical position (top or bottom) for your line, so the browser kinda just does anything.
Try adding
.headline:after {
top: 100%;
}
JSFiddle: https://jsfiddle.net/0p7948Lx/2/

Answer 2

I can't reproduce your issue in Firefox 60.8 on Debian. However, here is a somewhat simplified approach that works without position, transform and border. I find it a little more elegant.
Maybe it fixes the issue as well?
h2 {
width: 230px;
text-align: center;
}
.headline {
line-height: 1.5em;
}
.headline:after {
content: '';
display: block;
width: 100%;
height: 8px;
margin-top: 1em;
background: #64c800;
border-radius: 4px;
}
<h2><span class="headline">Contact</span></h2>