Form method “POST” not working in firefox

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ASP.NET

Javascript

HTML

The following code works in Chrome Version 47.0.2526.73 but on Firefox and IE 11 doesn't work. Any ideea why

HTML:

<form action="@Url.Content("~/Account/LogOn/")" method="post" id="login_form">
     <input type="text" id="username" name="username" placeholder="Username" value=""/>
     <input type="password" id="password" name="password" placeholder="Password" value=""/>
     <button id="btnLogin" class="btn btn-inverse pull-right" type="submit" onclick="OpenPage()" >Sign In</button>
</form>

JavaScript:

function OpenPage() {
    $('body').empty().load('../../../Content/LoaderHtml/loader.html');
    return false;
}

The code must call an ActionResult method from MVC

Only the onclick event get's fired in Firefox and IE (in Chrome the form also triggers the Controlller's method), if i remove the onclick event everthing works well in firefox and ie and chrome

I need to trigger the onclick event and also to post the form to server (The Chrome behaviour is ok, but how can i make this work even in firefox and IE)

Edit:
I've tried also

onclick="return OpenPage()"

and in javascript

return true;

Thanks for help!