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.
Kees
Downloaded Firefox from Cnet and received an Avira notice
regarding: TR/Agent.1002472.1[trojan] - I
made the mistake of clicking on "details" instead of on
"remove" and haven't seen an option to remove it since.

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