Margins work fine on chrome but not on firefox - css

Question

I'm learning basic CSS currently, I'm trying to create a little mockup product page that uses a grid system with 4 columns. It looks and works fine on chrome, but on Firefox it breaks like such. Here's how it looks like on Firefox:
I can't seem to figure out how to fix it, I've tried changing the margins/padding from various locations but to no avail, it's still messing up on Firefox.
Here's my CSS:
html {
font-family: Helvetica Neue, Helvetica, Arial, sans-serif;
background: #232526; /* fallback for old browsers */
background: -webkit-linear-gradient(to left, #232526 , #414345); /* Chrome 10-25, Safari 5.1-6 */
background: linear-gradient(to left, #232526 , #414345); /* W3C, IE 10+/ Edge, Firefox 16+, Chrome 26+, Opera 12+, Safari 7+ */
}
* {
box-sizing: border-box;
}
.menu {
background-color: #17534b;
position: absolute;
width: 99.23%;
height: 8vh;
}
ul.nav {
width: 100%;
}
ul.nav li {
list-style: none outside none;
display: inline-block;
width: 19%;
text-align: center;
}
ul.nav li a {
display: block;
text-align: center;
padding: 1.5em;
color: #ffffff;
letter-spacing: 1px;
font-size: 15px;
text-transform: uppercase;
text-decoration: none;
}
ul.nav li a:hover {
background-color: #000000;
color: white;
}
ul.nav li.li-ultimate{
text-transform: uppercase;
color: #eef2ff;
font-size: 25px;
}
section {
width: 100%;
padding: 0 7%;
display: table;
margin: 0;
max-width: none;
height: 100vh;
}
.welcome {
height: 100vh;
background-color: #ffffff;
}
.content {
display: table-cell;
vertical-align: middle;
}
.welcome h1{
font-size: 3em;
display: block;
color: black;
font-weight: 300;
}
.welcome h2 {
font-size: 1.5em;
display: block;
color: black;
font-weight: 200;
}
.productsheader {
color: #ffffff;
}
.productsheader h1{
text-align: center;
color: #ffffff;
font-size: 3em;
}
.products{
height:100vh;
}
ul.rig {
padding: 5%;
list-style: none;
font-size: 0px;
margin-left: -2.5%;
}
ul.rig li {
display: inline-block;
padding: 10px;
margin: 0 0 2.5% 2.5%;
background: #ffffff;
border: 1px solid #ddd;
font-size: 16px;
vertical-align: top;
box-shadow: 0 0 5px #ddd;
box-sizing: border-box;
-moz-box-sizing: border-box;
-webkit-box-sizing: border-box;
}
ul.rig li img {
max-width: 100%;
height: auto;
margin: 0 0 10px;
}
ul.rig li h3 {
margin: 0 0 5px;
text-align: center;
}
ul.rig li p {
font-size: .9em;
line-height: 1.5em;
color: #999;
text-align: center;
}
ul.rig.columns-4 li {
width: 22.5%;
}
Along with the relevant HTML:
<section class="products">
<div class="content">
<div class="productsheader">
<h1>
Our Proudly Presented Products
</h1>
</div>
<ul class="rig columns-4">
<li>
<img src="images/1.jpg"/>
<h3>Is-it-even-a-cup Cup - $100</h3>
<p>Life's true questions.</p>
</li>
<li>
<img src="images/2.jpg"/>
<h3>Flute Looking Cup - $150</h3>
<p>Looks like a flute.</p>
</li>
<li>
<img src="images/3.jpg"/>
<h3>Uterus Looking Cup - $350</h3>
<p>Modelled after a woman's uterus. Probably.</p>
</li>
<li>
<img src="images/4.jpg"/>
<h3>Artistic Cup - $1</h3>
<p>Artsy...ish...</p>
</li>
<li>
<img src="images/5.jpeg"/>
<h3>Ancient Cup - $500</h3>
<p>Or at least we think its old. Sure does look it.</p>
</li>
<li>
<img src="images/6.jpg"/>
<h3>Chinese Poetic Cup - $250</h3>
<p>The cup feels empty when you drink, but you're empty when you don't.</p>
</li>
<li>
<img src="images/7.jpg"/>
<h3>Dragonfly Cup - $25</h3>
<p>Pretty little dragonfly...On a cup.</p>
</li>
<li>
<img src="images/8.png"/>
<h3>FIFA World Cup Brazil - $50</h3>
<p>Technically a cup...</p>
</li>
</ul>
</div>
</section>
Are there any tips to also prevent this from happening in the future when I work on another css project?

Answer 1

ul.rig {
padding: 5%;
list-style: none;
font-size: 0px;
margin-left: -2.5%;
width: 960px; // choose your width like 960px
margin: auto; // auto work with display : block for centering
display: block; // block work with margin : auto for centering
}