Hi dyte,
It works fine for me, I just add a simple iframe like below in the page and test it.
I test on firefox 3.6.10 on vista.
<iframe src="http://www.ajaxuploader.com/Demo/select-multiple-files-upload.aspx"
width="1000" height="500"></iframe>
Which version of firefox you are using? Do you have the same issue on another browsers/pc?
Keep me posted.
Regards,
ken