why not just rearrange your nav,section, header to be like (using your current css)
<div id="mainContainer">
<div id="divA"></div>
<div id="divB"></div>
<div id="divC"></div>
</div>
but if you just want to have it with that arrangement check my fiddle http://jsfiddle.net/4Jpts/9/
what i basicaly did was to delete the left float from divA and clear what was on the right of divC