I have discovered a strange bug which only appears in Firefox.
I have created a cut down demo:
The menu "Skip Navigation and Sitemap" should appear in the top right hand corner of the page. Structurally they are created after the site name and slogan.
As absolutely positioned items should be placed in relation to their containing block (in this case the <div id='topsection'> div) then the way IE, Opera and Netscape renders it are correct.
Strangely Firefox renders it by placing it on the same line as the slogan, rather than in the top right hand corner. Is this a known bug?