Did you check for interference with other extensions?

Try a new profile with only that extension installed.

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer some files from an existing profile to the new profile, but be careful not to copy corrupted files.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

I tried your suggestions above but the Download Helper only works on a couple of websites like YouTube but does not work on the site it has always worked on before the update. the site is: http://www.free-tv-video-online.me/internet/

Unfortunately it is not much use to me if it doesn't work on that site as that is the one I used it on the most since I watch all TV programs on the Internet and don't own a TV.

Any other suggestions would be appreciated.