Copy this code without editing it and save with a new filename because it shows what I think you are trying to do:
<!DOCTYPE html>
<head>
<title>bo newsletter 6 slots</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<style type="text/css">
body{
background:#FFF;
margin:0;
padding:0;
}
.header{
background-attachment: scroll;
background-clip: border-box;
background-color: rgba(0, 0, 0, 0);
background-image: url(/images/headerBg.png);
background-origin: padding-box;
background-size: auto;
border-collapse: separate;
color: rgb(26, 77, 128);
display: block;
float: left;
font-family: Arial, Helvetica, sans-serif, Calibri;
font-size: 14px;
font-weight: bold;
height: 130px;/*118px;*/
line-height: 18px;
width: 950px; background-color: orange;
}
.logo{ background-color: yellow;
border-collapse: separate;
color: rgb(153, 153, 153);
display: block;
float: left;
font-family: Arial, Helvetica, sans-serif, Calibri;
font-size: 15px;
font-weight: bold;
height: 80px;
line-height: 18px;
margin-bottom: 0px;
margin-left: 5px;
margin-right: 0px;
margin-top: 40px;
width: 295px;
}
.menucon{ background-color: skyblue;
border-collapse: separate;
color: rgb(153, 153, 153);
display: block;
float: right;
font-family: Arial, Helvetica, sans-serif, Calibri;
font-size: 15px;
font-weight: bold;
height: 118px;
line-height: 18px;
width: 450px;
}
.social{ background-color: lime;
/*width: 50px;*/
display: block;
height: 80px;
line-height: 18px;
}
</style>
</head>
<body>
<div class="header">
<div class="logo"><a href="index.php">Logo</a>
</div>
<!--<div class="social">test</div> coded after right div-->
<div class="menucon"><!--not menuCon with capital C-->
<!--<div></div>-->
<!--<div></div>-->
<div id="menu">
<ul class="sf-menu">
<li class="home"><a href="index.php"></a></li>
<li><a href="knowledgebase.php"><span></span>F.A.Q's</a></li>
<li><a href="announcements.php"><span></span>News</a></li>
<li><a href="clientarea.php"><span></span>Client Portal</a></li>
<li><a href="contact.php"><span></span>Contact Us</a></li>
</ul>
</div><!--end of #menu-->
</div><!--end of .menucon-->
<div class="social">test<br />test in .social div<br />test in .social div</div>
</div><!--end of .header-->
</body>
I've given the divs colors so that you can see the arrangement.
If you want two floated divs (one left and one right) with a div in the middle, code the float left and float: right divs first in the markup, then the middle div and the middle div .social with a lime color fills all the space between the left and right divs. Note that .social is without a width and in fact overlaps the float left div but the text content will start at the side of the float: left div. If you give the middle div a width, it will drop down unless you code that with float: left as well and put it after the first float; left div.
Be careful to use the same case for style and markup, you had .menucon with a lower case c in the markup and .menuCon with a capital C in the style.
You had two empty divs in .menucon which I have deleted.