I'll suggest you download and install Malwarebytes
and to run a full scan of your pc. Second,do you have the latest version of the browsers? Also I know when I was using Norton,after downloading a bigger file,I should wait for a 5-6 seconds to scan the file that I'm downloading. It could depend also on your internet connection,if you have slow internet connection. And the last thing try to start Mozilla Firefox in Safe Mode,and try to download that file from there,maybe some ad-don is causing that.