I am creating my first mock up in fireworks. How come when I preview either in screen or in browser al the gradients look bad? Is it because they are not flatened yet? Do I need to make some adjustments for it to come out better in prewiew or will that come out better later? Like I said I'm new so any help would be appreciated. One more question. When I make a slice and new state then a rollover effect is there a way NOT to see the green boxes over the objects while I'm editing? Thanks.
I suspect you haven't assigned file export types to your slices yet. Select one of the slices containing a gradient by clicking on it, and then open the Optimize panel. Click on the Preview button at the top of the document window. In the Optimize panel, set the format for the slice to JPEG - high quality. It should look quite good in the preview window now, and just as good when you do a browser preview.
You can turn slice preview off by clicking on the button circled below.
Adding to Linda's comments, Fireworks default optimization setting is GIF (no idea why). Usually, GIF format does a terrible job of optimizing gradients. Slice your design as needed, optimizing the slices, and set the base optimization to jpeg or PNG. You change the base optimization setting by making sure no other slices are selected, then changing the setting in the Optimize panel.