Hi,
I have a test on your website on the IE and firefox, the result as you said, the login button can work on the fiefox, but not on IE.
It is very weird! But i create a new page, and drag a login control on the page, the login button works on the firefox and IE.
But after review the source code of the login page, we find there seems exists two form tags on the login page:
1. <form name="aspnetForm" method="post" action="login.aspx?ReturnUrl=%2fDefault.aspx" onsubmit="javascript:return WebForm_OnSubmit();" id="aspnetForm">
2. <form action="~/SearchResults.aspx" id="searchbox_017601747286395297899:nl-qqt5da6y">
You need to delete the second form tag or remove the runat=sever of the second form tag, because you can have more than one form tags, but only one form runs at server on one page, or else, the page cannot work fine at the render time.
If there is cannot help you solve your problem, I'a afraid that you need to post the login page's code here for further help.
Hope it helps