If you are downloading it from the official Mozilla add-on repository, then this is a false positive or overzealous antivirus protection and the file is not truly infected with a virus. As you can see, the threat name says "Suspect". Some detection rule in your AV program is responsible for this. A quick search here https://community.mcafee.com/ suggests it's a "TIEM" rule in effect. You're better off asking there how to change the responsible rule or add an exclusion for the suspected file.