Per, I have just tested upload through Firefox on my WinXP computer and it works just fine. There is no problem with the site.
Please, provide some more information as to what exactly happens when you attempt to upload, so that we can help you resolve the issue. Does it work with another browser? Perhaps your connection is not stable?
Actually, I'm having also some problems uploading via FF / XP.
The difference is that after the upload ends, the image (tried eps and jpg) does not showp up on the editing page (just an image icon). Already cleared cache, history, cookies. Then tried IE.7, and everything is ok, files uploaded ok and images can be seen. Then, back to FF just to check if the inittial problem was still standing, but no, the upload process with IE made it through.