The only thing I can tell you is to check your code, make sure you don't have more then one reference to mm_menu.js.
When you export the html file from fireworks, can you open that and view the menu? Or is it only when you try to put the menu code in a page that you built?
If that is the case then the problem lies in your path.
Open FW, open your menu png, then export to the same folder that houses your html file that you want to have the menu. Once you open the file you want to put the menu in, find your spot and click the FW button on the toolbar, thiss will insert your images and javascript and proper path to the mm_menu.js.
If that still doesn't work, you may want to check your java logging settings. you may have an error in your browser settings.
Hope that helps