I'm saying that it works well in every other application except firefox, so I think it's related to firefox. As a user, I can write in the fat32 partition in all the applications (konqueror, nautilus, kget, gaim, amarok, even amule and azureus) but not with firefox. It can write because it creates an empty file but instead of filling it, the download ends immediatly and the file is left with a size of 0 bytes.
I'm using the latest version and it's still the same. It's happening since the first time I installed Edgy (it was in beta at the time). It didn't happen with Dapper and I haven't tested Edgy yet.
I have tried disabling and uninstalling all the extensions, even deleting the .mozilla directory and it still happens.