[Problem solved by changing export options for HTML tables. Correct options in this case are "Space width" 1-pixel transparent spacer and "Empty table cell contents" Spacer image. Previously I had nested tables and that didn't work with buttons.]
I have a prototype web page in Fireworks. It has some rectangles on a color background and all is exported ok as "HTML plus images". But when I change these rectangles into Button symbols, the export layout is broken - there are white horizontal spaces (canvas color is white) across the whole page above and below each button. Nothing elses changes - only converting rectangles into buttons breaks the layout. Is there any method to control it, any checks to make etc?
By the way, reverting those buttons back to rectangles and adding hotspots fixes the problem, keeping navigation. But anyway, buttons are a bit faster, so it would be good to use buttons symbols and still be able to export to HTML.
In addition to the suggestion about using 1 pixel transparent spacers, if you are designing on a transparent canvas, try changing it from transparent to a color that matches the background color against which you have placed the buttons.