Firefox cannot download
It should be sufficient to delete the file Downloads.rdf:
Windows Users
Enable hidden files in My Computer> Control Panel> Folder Options> Select from the list "Show hidden files and folders"
You can then delete Downloads.rdf in:
C: \Documents and Settings\[User-Name]\Application Data\ Mozilla\Firefox\Profiles\[name].default\
Then turn off the display of hidden files in My Computer> Control Panel> Folder Options> Select from the list "Do not show hidden files and folders"
GNU/Linux Users
/home/[User-name]/.mozilla/firefox/[name].default\
• The name defines nothing, for example 06hy2yd9.default