Usually it works quite well with Firefox. But still not for me this time. I will try IE now... thanks for your answer and the hint! (Even if it doesn't work...!)
Edit: yes, it worked ;-)
I now know why too: I have blocked some scripts with Firefox... It must have something to do with Google or Facebook scripts which I have blocked both (I don't like near-monopolists... thus I don't use them) - no idea why mediafire uses them but who knows....