alt131 - 10:39 am on May 27, 2011 (gmt 0)
That's the key piece of information for me, so hopefully we can get a solution without needing more code.
it has a scroll bar at the bottom and only about 3/4 is viewable within the screen
Browsers vary on their error handling, so good on you for validating - but in this case I suggest double-check the css and the html (especially the html) just to make sure ;)
Then look for elements that have a width/padding/margin combination that would make them wider than the viewport width. Also check for positioned elements that have been shifted from their actual position using a negative left or margin-left.
As you are using firefox, the firebug extension makes it really easy to check dimensions in the "layout tab". Otherwise set a border/background on each element so you can see where it is being drawn.