I am working on a site in D that was exported (css and images) from fireworks.
Unfortunately, that is the source of your problems. The HTML code that is written by any graphics application (or any wizard for that matter) is unacceptible for use on the web. The feature is designed and intended purely for production of prototype layouts, i.e., suitable for demonstrations in a controlled environment.
The gaps are the result of the interaction between a) changes in the shape of the contents of your table layout, and b) the fact that the tables are built using col- and rowspans (i.e., merges and splits). Please read this article to learn more about how merges and splits degrade the stability of your page - http://www.apptools.com/rants/spans.php
The solution is to rebuild the page, doing all of your coding in DW and only using the graphics from FW. These links will help you do that -
HTML & CSS Tutorials - http://w3schools.com/
How to Develop with CSS?
Learn CSS positioning in 10 Steps
PRE-BUILT CSS LAYOUTS
For rock solid, commercial products that perform well in all browsers/devices, visit Project Seven:
Not Just a Grid CSS Framework (free)
Ultimate Multi-Column Layouts (free)
EZ-CSS Templates (watch the screencast to see how it works)
Dreamweaver CSS Templates for beginners (free)
New DW Starter Pages (free)