Bootstrap 3：布局在IE9中破碎，无法弄清楚原因

Question

首先是链接：http://isotopethemes.com/extenso/footer-15.html

仅限IE9中的页脚断开，在所有其他浏览器中都能正常工作。花了好几个小时试图搞清楚。 IE不善于发现问题。

HTML代码：

<footer id="footer-15">
    <div class="footer-15-main">
        <div class="container">
            <div class="row">
                <div class="footer-15-item col-lg-3 col-md-3 col-sm-3">
                    <h3 class="footer-15-heading"> Connect With Us </h3>
                    <p> Follow us on social media </p>
                    <div class="social">
                        <a href="#" class="button">
                            <i class="fa fa-facebook"></i>
                        </a>
                        <a href="#" class="button">
                            <i class="fa fa-twitter"></i>
                        </a>
                        <a href="#" class="button">
                            <i class="fa fa-linkedin"></i>
                        </a>
                        <a href="#" class="button">
                            <i class="fa fa-google-plus"></i>
                        </a>
                    </div><!-- /.social -->
                    <div class="clearfix"></div>
                </div><!-- /.footer-15-item -->
                <div class="footer-15-item col-lg-3 col-md-3 col-sm-3">
                    <h3 class="footer-15-heading">Newsletter Signup </h3>
                    <p>Signup for our newsletter</p>
                    <form>
                        <input type="text" class="footer-15-input" placeholder="Enter Your Email...">
                        <button class="btn">
                            <i class="fa fa-search"></i>
                </div><!-- /.footer-15-item -->
                <div class="footer-15-item col-lg-3 col-md-3 col-sm-3">
                    <h3 class="footer-15-heading">Important Links</h3>
                    <ul class="footer-15-list links">
                        <li><a href="#">More About Us </a></li>
                        <li><a href="#">Checkout Our Works</a></li>
                        <li><a href="#">Our Locations</a></li>
                    </ul><!-- /.footer-15-list -->
                    <div class="clearfix"></div>
                </div><!-- /.footer-15-item -->
                <div class="footer-15-item col-lg-3 col-md-3 col-sm-3">
                    <h3 class="footer-15-heading"> Contact Us </h3>
                    <ul class="footer-15-list contact">
                        <li>
                            <i class="fa fa-map-marker"></i>
                            221B Baker Street,
                            London.
                        </li>
                        <li>
                            <i class="fa fa-phone"></i>
                                +07745 352 515
                        </li>
                        <li>
                            <i class="fa fa-envelope"></i>
                            <a href="#">contact@isotopethemes.com</a><br>
                        </li>
                    </ul><!-- /.footer-15-list -->
                    <div class="clearfix"></div>
                </div><!-- /.footer-15-item -->
            </div><!-- /.row -->
        </div><!-- /.container -->
    </div><!-- /.footer-15-main -->
    <div class="footer-15-bottom">
        <div class="container">
            <div class="row">
                <div class="item col-lg-12 col-md-12 col-sm-12">
                    <p> &copy; Copyright 2014 Extenso. All Rights Reserved. Designed by <a href="#">isotopethemes</a>
                </div><!-- /.item -->
            </div><!-- /.row -->
        </div><!-- /.container -->
    </div><!-- /.footer-15-bottom -->
</footer><!-- /#footer-15 -->