I am experiencing the same thing. In FireFox, I have AjaxUploader in an iframe that is contained inside an ajaxToolkit:TabPanel. The Upload button stays gray/inactive. The CuteWebUI_AjaxUploader_OnInitialize event does fire, but the button remains grayed out. If I navigate to the uploader page (outside the Iframe) then AjaxUploader works fine - button is available. Inside IFrame it remains gray. Since the Iframe is contained within the ajaxToolkit:TabPanel I can't make it the first html element after the body tag!
FireFox: v 3.6.17
AjaxUploader: v 3.0.0
OS: Windows Vista
Any help would be greatly appreciated!