CSS 3-column header margin question

I am working on a template for joomla , not that it matters in the end.

I am trying different 3 column layouts....

In this particular layout my header (#mast ID) has a bottom margin in FF and I am not sure why.

I am also curious why the left column will flow behind the text but the right margin does not.....i never quite understood that.

the template test site is at www.mychemistrytutor.com/index.php

it is hidden behind a .html file

thanks a bunch

 

You should set margins and paddings.
for the gap use
ul#mainlevel-nav{margin-bottom:0;}