The following technique is also known as Full Browser Flash:

Set both the width and height of your SWF to 100% in your SWFObject definition

Include CSS to get rid of any default margins/padding and set the height of the html element, the body element and the entire chain of block level HTML elements that your SWF will be nested in to 100%, because Firefox (or: any Gecko based browser) in standards compliant mode (or: using a valid DOCTYPE) interprets percentages in a very strict way (to be precise: the percentage of the height of its parent container, which has to be set explicitly):

<style type="text/css" media="screen">

html, body, #containerA, #containerB { height:100%; }

body { margin:0; padding:0; overflow:hidden; }

</style>

Manage the scaling and alignment of your SWF and the positioning of your SWF's elements, within your ActionScript code, e.g.:

stage.scaleMode = StageScaleMode.NO_SCALE;

stage.align = StageAlign.TOP_LEFT;

stage.addEventListener(Event.RESIZE, resizeHandler);

function resizeHandler(event:Event):void {

// center stuff

}