PHP - Moved: Php & Firefox 4 Problem

I developped a cms system to upload images with php to a mysql database and a upload folder on the server.
Firefox version 5.0.1 for MAC and earlier have no problems with uploading through the script. But after that version it is impossible to upload images bigger then 250kb. The page where the customer chooses his image for upload stalls and the form is not sent to the actual upload script, which should validate and process the image. This problem only occurs in Firefox, all other browsers work just fine!
I tried to run the same script with Firefox on a PC and everything works fine. So it is a Mac related problem.

You can try for yourself to see what happens in the following basic script (if you don't get a succes message it means that you are still hanging on the image_upload page and nothing has happened) :
http://siteways.net/test_upload/image_upload.php

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