Two column layout with header - RH column overlapping into Header area

I have used a DW Layout css - twoColFixLtHdr.css.
The first item in the right hand column div(maincontent) is div flattext. I have put a background colour on this which is overlapping the menu bar in the header div, in Firefox.
In IE it is ok. 

I have tried everything I can think of and wasted two days on this problem.
Help would be so appreciated. (I need to know what to try and why it is happening)
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Try adding the following to #flattext:
float: right;
margin: 25px 0px 0px 0px;

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You haven't cleared the floats used for your navigation, therefore the floated content its floating with the navigation. I would recommend changing

<div id="sidebar1">...</div>
<div id="mainContent">...</div>

to something along the lines of

<div id="content" style="clear:both;">
  <div id="sidebar1">...</div>
  <div id="mainContent">...</div>

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BigpollyAuthor Commented:
I have wasted so much time with this and in the end it was so simple. I can't thank you enough!
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