I have a button element with a custom background (image+color) and no borders except for a 2px border-bottom (and a bunch of other properties --code below) which renders quite differently in Firefox and in IE8. The problem is, this is a work for a company that uses IE8 as their only browser, so it's important that the button renders well in IE8.

Here's a visual comparison between the two:

My question here is not about the padding difference (I'm looking into that), but about the weird border that is visible on IE8 in addition to the regular border (border-bottom).

Can anyone explain to me where it comes from and how to get rid of it?
Thanks in advance.

Here is the HTML code:

<button class="btn" id="c_edit">
    <span>Annuler</span>
</button>

And here is the CSS:

.btn {
    display: inline-block;
    margin: 0 0 7px 5px;
    padding: 0;
    color: #ddd;
    font-size: 14px;
    font-family: FrutigerLTStd55Roman, sans-serif;
    text-decoration: none;  
    border: none;
    border-bottom: 2px solid #222;
    background-color: #999;
    background-image: url('img/btn_bg.gif');
    background-position: 0 bottom;
    background-repeat: repeat-x;
    cursor: pointer;
    transition: all .5s ease-out;
}
.btn span {
    display: inline-block;
    margin: 0;
    padding: 8px 10px 6px 40px;
    background-color: transparent;
    background-position: 4px 0;
    background-repeat: no-repeat;
}