Hi, Welcome to Sitepoint 
You basically just need to add a right position to that absolute element to push it into place. Set position:relative on the container and then the element will be placed to the right of the container and not the viewport.
You have added float to the body which is killing firefox so remove it 
You don;t need the dropshadow div as the image can be placed on the container anyway.
Pay attention to the changes in bold below:
Code:
html,body{margin:0;padding:0}
body {
font:13px "trebuchet MS";
color:#898b8c;
background-color:#fff;
/* float removed*/
}
#container {
margin:0px auto;
width:1000px;
padding:0 5px;
background:url(http://members.incentiveusa.com/awardpages/propel5/test/css/images/background_dropshadow.jpg) repeat-y 50% 0;
overflow:hidden;
position:relative;
}
.main_right_login_details {
position:absolute;
width: 240px;
margin:0px 0px 0px 35px;
background:url(http://members.incentiveusa.com/awardpages/propel5/test/css/images/login_panel.gif) no-repeat;
height:110px;
font-size: 10px;
top:0px;
right:5px;
}
You've gone a bit overboard on your comments and there's no need to have all those start comments as the class is information enough.
You can add comments to the end of a container but only do it for the main structural containers and not all container. You have more comments than html and it actually makes it harder to work with. Only comment where necessary especially as comments trip older versions of IE up all the time.
No need for this:
Code:
<p class="clear"><br />
<br />
</p>
Use overflow:hidden on the parent or if you want visible overflow then use the clearfix method (see faq on floats - see my sig).
Don't wrap divs around uls when there's no need. The ul is a prefectly good container on its own.
The heading inside your ul needs to be inside a list item as all content must be in the list item in a ul structure.
Code:
<ul class="mid_list">
<li>
<h1 class="headerTitle">Leaderboard</h1>
</li>
Don;t go straight form a block level to an inline element if you can help it.
Code:
</ul>
<span class="read_more"> <img src="http://members.incentiveusa.com/awardpages/propel5/test/css/images/Down_Arrow.gif" alt="" border="0" /> <img src="http://members.incentiveusa.com/awardpages/propel5/test/css/images/Up_Arrow.gif" alt="Up" width="18" height="16" border="0" /> </span>
The span should really be a p element as it is not part of the same sentence as the ul.
Don't use breaks to make space:
Code:
<h4 class="footerCopyright">Copyright © 2011, ASIM. All Rights Reserved.<br />
</h4>
Just use css and add the extra space you need with padding or margin where appropriate. Breaks are never used just to make space. They are used for natural line-breaks such as in poems, addresses, and between form controls.
Hope that helps
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