When I disable your stylesheet.css, the page displays properly as far as layout.

That leads me to believe the problem is in the stylesheet.css. I do find several instances where you have "double-dipped" calls. TABLE.header on line 109 has background as a color, then an image. TABLE.headerNavigation on line 122 has both background and background-color. You also have used some labeling that may cause some problems in some browsers. At line 210 and several more after, you used .column_box_spacer_left when naming convention generally dictates .coulmnBoxSpacerLeft. Get yourself a copy of TopStyle Lite and take a good look at the stylesheet.

BUT, if you look at the source file after cleaning the html in FireFox, everything displays perfectly. I do see several instances of using
###px
in td width and height calls versus just
###
.

I see lots of <span which leads me to assume that you may have been using a file editor that throws these tags in just for fun. They are NOT fun when they start messing with HTML validation as some of yours have.

Bottom line is that I feel you have several small problems in the stylesheet and several problems in the source files.

Fixing the improper widths in the source files would be what I'd work on first. That's probably causing the wrap in the table-based layout. Makes you see why ZenCart is trying to get away from table-based layout.

Best of Luck. Let us know if we can help more.