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).

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
}

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).

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
}