Alignment of third level navigation in centered navigation menu out in IE - html

Question

I have created a drop-down menu which works fine in Firefox but the alignment of the third level dropdown menu is out in IE, and I can't work out why.
The HTML:
<nav class="main-navigation clearfix">
<ul class=" main-nav-ul">
<li>Main Link 1</li>
<li>Dropdown
<ul>
<li>Level 2 Item 1</li>
<li>Level 2 Item 2
<ul>Level 3 Item 1
</ul>
</li>
<li>Level 2 Item 3</li>
</ul>
</li>
<li>Main Link 3</li>
</ul>
</nav>
And the CSS from styles.less file:
.main-navigation {
text-align: center;
font-family: #body-font;
}
.main-navigation ul {
li {
display: inline;
text-align: center;
position: relative;
a {
color: lighten(#dark-color, 10%);
display: inline-block;
font-size: 14px;
font-weight: bold;
height: 42px;
line-height: 42px;
margin-right: 14px;
position: relative;
&:hover {
color: black;
}
&::before {
content: '|';
position: absolute;
font-size: 10px;
right: -1em;
top: -1px;
color: lighten(#dark-color, 50%);
}
} //end a
ul {
opacity: 0;
filter: alpha(opacity=0);
visibility: hidden;
-webkit-transition: opacity 0.2s ease-in;
-moz-transition: opacity 0.2s ease-in;
-o-transition: opacity 0.2s ease-in;
transition: opacity 0.2s ease-in;
background: #light-text-color;
border: 1px solid darken(#light-text-color, 10%);
border-bottom: 0;
position: absolute;
left: -49px;
li {
float: none;
white-space: nowrap;
position: relative;
a {
padding: 0 14px;
margin-right: 0;
border-bottom: 1px solid darken(#light-text-color, 10%);
&::before {
content: '';
}
} //end a
ul {
position: absolute;
left: 92px;
top: -15px;
opacity: 0;
filter: alpha(opacity=0);
visibility: hidden;
-webkit-transition: opacity 0.2s ease-in;
-moz-transition: opacity 0.2s ease-in;
-o-transition: opacity 0.2s ease-in;
transition: opacity 0.2s ease-in;
} //end 3rd level ul
} //end 2nd level li
} //end ul
&:hover > ul {
opacity: 10;
filter: alpha(opacity=100);
visibility: visible;
}
} //end li
} //end main-navigation ul
I am using bootstrap CSS as well. I'm still quite new to CSS so my apologies if coding isn't so good!
Thanks