This sounds like a very common problem that happens after you have installed and then deleted Speed Download (or other programs that work to make downloads faster). Did you by any chance install the program Speed Download and then later delete the program from your Mac?
If so, you'll probably find that you can download files by Control-clicking (or right-clicking) on a download link and choosing "Save Linked File As" from the contextual menu. In that case, download the Speed Download installer on
this page of their web site and open the .zip and resulting .dmg files. Inside the Speed Download 5 folder you'll find an uninstaller application. Run that to properly remove Speed Download and all of its supporting files from your Mac.
Message was edited by: Rachel R
Message was edited by: Rachel R