Pode relatar melhor seu problema? vc faz um download e nem não aparece na lista de download do próprio firefox é isso??? qual versão do navegador Firefox vc usa?

It is possible that anti-virus software is corrupting downloaded files or is otherwise interfering with downloading files.

Try to disable the real-time (live) scanning of files in your anti-virus software temporarily to see if that makes downloading work.

See:

• http://kb.mozillazine.org/Unable_to_save_or_download_files
• browser.download.manager.scanWhenDone

• https://support.mozilla.org/kb/Unable+to+download+or+save+files