Basically same as archived thread "download" (https://support.mozilla.org/en-US/questions/1328091). I've tried all the suggestions; none worked. Might have same problem in Safari, but never with Chrome. Also, only started with Firefox 2 days ago. Refreshing page sometimes helps, usually doesn't. It's like trying to start a fire with a flint spark. It either "goes" or it doesn't. Very weird. Btw, when it goes, link turns purple. When it fails, link stays blue.