Mozilla Firefox, Event.RESIZE and stage.stageWidth

I have strange problem with stage.stageWidth when I’m scalling browser window. In IE everythings works fine, but Firefox giving me 418px even where browser size it’s smaller!! I need exact width of the browser, even when is 100px only. Code is very basic and it’s on timeline:
import flash.display.StageAlign;
import flash.display.StageScaleMode;
stage.align = StageAlign.TOP_LEFT;
stage.scaleMode = StageScaleMode.NO_SCALE;
function onResize(event:Event):void
var sw:Number = stage.stageWidth;
var sh:Number = stage.stageHeight;
sw_txt.text = String(sw);
sh_txt.text = String(sh);
where sw_txt and sh_txt are 2 dynamic TextFields on the stage. I have no clue why it’s like that. If anyone could check it and put the answer I would be more than grateful. Greetings to all ActionScript coders!