Just ran into this weird problem with a user using Mac OS X. This user always had a failed upload. The form uses a regular "input type=file". The user could upload using any browser except Firefox 3 on his Mac. Only this particular user was seeing this error.
Obviously, the problem is only with this one particular user.
User corrected this weird problem by recreating their FireFox profile. How to manage FireFox profiles
I imagine a re-install of FireFox would have corrected the problem as well.
I imagine a re-install of FireFox would have corrected the problem as well.
Profile related problems cannot usually be solved by re-installing Firefox since reinstalling (or upgrading) would re-use the same "damaged" profile.