Add the following to your style sheet. If it's too much, then just add it to the parent of the table.

.container { clear: both; }

<div class="container" style="clear: both">
<table width="850"…

You should also address the markup and CSS errors in your site, and add a site icon.

• Result for baxterdentalclinic.com | Unicorn - W3C's Unified Validator
• How to add favicon to your website | Adobe Contribute Blog

It seems the <li> elements in ul#nav are height:38px, while the ul#nav itself is height:37px tall (per 360dental.css). This is causing the bad float behavior.

In nav.css, on line 10, you actually do have height:38px but Firefox is not reading it. Hmm... okay, there is a semicolon missing at the end of the previous line. So, add a semicolon to the end of line 9 in nav.css and it should be fixed.

Good catch Jefferson!

Adding this missing semicolon in the Syle editor immediately fixes this issue.

#nav {
	background: url(full-header.gif);
	height: 38px;
	width: 850px;
	margin: 0 auto;
}

Note that the Web Console (Firefox/Tools > Web Developer) can be used to find such errors.

Expected color but found 'height'.  Error in parsing value for 'background'.  Declaration dropped. nav.css:10
Expected ':' but found '850px'.  Declaration dropped. nav.css:11
Error in parsing value for 'background-position'.  Declaration dropped. nav.css:65
Expected ':' but found '850px'.  Declaration dropped.

There is also a missing colon with the width 850px; rule in 360dental.css

#nav {
	
	height: 37px;
	width: 850px;
	margin: 0 auto;
	background-image: url(/Images/page_images/nav.png);
}

Thank you!!! Look what happens when you work too late! Simple fix. Much appreciated.