Re: downloaded firefox
Deleting that file should be sufficient. But you might like to upload to virustotal.com and see if other antivirus program agree with Avast about it containing a virus.
Wiping the empty sectors shouldn't be necessary, although it won't harm either if you have a program to do it.
By the way, it escapes me why anybody should download Firefox from CNET when there is www.mozilla.org to download it from.