First time post but long time viewer -- I built a nav in Fireworks. It checks out in the browser preview. When I insert it into Fireworks as an image object, the 1000 px X 35 nav exceed the nav <div> by like 40 px. What?
If you find a browser that's the same as the Fireworks browser preview, it'll work. Otherwise design it by hand to fit in the div. (And stop thinking fixed-width divs - you can't know in advance what size window any particular user is going to have. Design your sites so that content is just content, not size. Markup is what it is, not what it looks like,)