Ubuntu font causes weird selection borders in Firefox

I am using the Ubuntu font from google fonts in my project. Unfortunately, when using fonts, Firefox does not display the right border of the selection / drop-down menus.

I created a script, test it using Firefox.

Corresponding code

<link 
    href="http://fonts.googleapis.com/css?family=Ubuntu" 
    rel="stylesheet" type="text/css" />

<style>
    input, textarea, select, span
    {
        font-family: 'Ubuntu' , sans-serif !important;
        font-size: 13px;
    }​
</style>

<span>Some text to show that the font is indeed in use</span>
<br />
<select name="testSelect" id="testSelect">
    <option value="1">1</option>
    <option selected="selected" value="2">2</option>
    <option value="3">3</option>
    <option value="4">4</option>
    <option value="5">5</option>
    <option value="6">6</option>
    <option value="7">7</option>
    <option value="8">8</option>
    <option value="9">9</option>
    <option value="10">10</option>
    <option value="11">11</option>
    <option value="12">12</option>
</select>​