It is possible that your anti-virus software is corrupting the downloaded files or otherwise interfering with downloading files by Firefox. You can try to disable the real-time (live) scanning of files in your anti-virus software temporarily to see if that makes downloading work. See also http://kb.mozillazine.org/Unable_to_save_or_download_files ---- Your above posted system details show multiple Java Console extensions. You can uninstall (remove) the Java Console extensions and disable the Java Quick Starter extension, you do not need them to run Java applets. See http://kb.mozillazine.org/Java#Multiple_Java_Console_extensions See also http://www.java.com/en/download/help/quickstarter.xml - What is Java Quick Starter (JQS)? What is the benefit of running JQS? - 6.0 Disable the Java Quick Starter extension: Tools -> Addons -> Extensions Control Panel -> Java -> Advanced tab -> Miscellaneous -> Java Quick Starter (disable)