link works okay for me. you can google validate xhtml and it'll give you a link to the W3C site. That will tell you where your webpage has errors (or you can use firebug on firefox). By validate, I mean that it will look at your page as it is read by a browser and tell you if there are any technical mistakes, like open tags, it seems to be a common issue with firefox, it doesn't seem to play well with invalid pages. It's a common recommendation on these boards and one I use myself. It can be time consuming the first time or two because you have to find the incorrect lines on the validation page then go into the theme editor, pick the right file and edit appropriately.
Based on a quick glance over the validation page, it looks like you may have an open DIV in your main index. So you can look in your main index and see if you have more <DIV> than</DIV> and if so make the correction. If your not terribly familiar with how they work, I would back uo your theme first.
The more I think about it, viewing your page with firefox with firebug installed may not be a bad idea. It should show you what firefox is seeing.
Sorry the directions aren't totally precise and certainly not concise but it's code so it's a little more lucid than that. I hope that helps to some degree.