I have the problem too. I tested about permissions, but it's not the
problem. The problem is with Firefox. I tested with other browser, such
Epiphany, and it was ok.
The problem: cannot save when i put the ftp address directly in the
browser, such http://www.lidec.futuro.usp.br/envio/animagimp.pdf
Cannot save as well when I try to save files from e-mails, for example
The problem does not occur when I firstly open the file with other program, such adobe reader, and then save the file.
If I click directly in a link with right button, and I choose to save, it's ok.
Also, the problem does not occur when I save in my home directory. The problem is for FAT particion.
I hope this way you have enough information.