Looks very good. Seems to be more than 100% wide, though. It produces a horizontal scrollbar at the bottom. More importantly: It throws tons of warnings (errors, for validation purposes) when checked for tidiness, some of them related to <div>s not closed properly or in a wrong order. That's very likely to break your layout at some point. You may want to get the HTML Validator extension for Firefox from
https://addons.mozilla.org/firefox/249/. It's extremely helpful for sifting through malformed html code. Also, once you're done, to enjoy the fact that yours is clean and so much out there terribly dirty.
Btw.: Since you're obviously caring about standards compliance and "separation of structure and presentation," if it displays "correctly" in IE6 and not in Firefox, then that's most probably because it does NOT display correctly (i.e., how the standards demand it should display) in IE6. IE7 has come a long way toward compliance, meaning it displays many things more like Firefox than like IE6. But I guess that, to reach the full range of browsers, including Opera, Safari and the Linux browsers, it's a better idea to make it work with the Fox, and then see how to fix it in IE6.
Monty has already pointed to positioniseverything.net. That's really an indispensable resource for all clearing and floating purposes. They have a really nifty pagemaker there which produces a series of layouts--fixed width, fluid, n number of columns, headers, footers, etc.--according to your specs, and with a variety of browser specific hacks already in it. It's at
http://www.positioniseverything.net/articles/pie-maker/pagemaker_form.php. Might be interesting for you to have an eye into that, even though yours seems to be mostly working now, the problems with the divs not withstanding. I think I've seen something similar somewhere else but don't remember where.
Nothing against JMorris's solution, but while you're at it, you may consider swapping the main column with the left column, so the content comes before the navigation. Good for small devices and search engine optimization.
