This issue has puzzled me for months till today. I finally found out the cause from here. So to make the swf file display, I'll have to define height for all the elements that will parent the file. ie:
<html>
...
<body>
...
<div id="parent">
...
<div id="child">
[swf object]
</div>
</div>
...
</body>
...
</html>
For this to work in Firefox, I need to define following css style:
html, body, #parent, #child {height:100%}
