FireFox ignores the size of Flash if it is set by percent. FireFox renders HTML strictly based on Doctype when Doctype is HTML 4.0, XHTML 1.0 or later version.

I had to code HTML with this type of Flash that other person made, and faced this problem. I didn't know this old issue.

The solution is adding size 100% for parent elements.

<style type="text/css">
<!--
html,body { height: 100%; width: 100%; }
-->
</style>

You also need add elements outside of Flash tags.

You can see a description about this issue on Adobe Developer Connection page.

http://www.adobe.com/cfusion/communityengine/index.cfm?event=showdetails&postId=3644&productId=1&loc=ja