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

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

You can try to set the browser.download.manager.scanWhenDone pref on the about:config page to false.

• http://kb.mozillazine.org/about:config

Thanks for answer but I wont get answer - how disable on all time this dialog box: not show message anytime

browser.download.manager.scanWhenDone set to false