You likely downloaded a small online Firefox stub installer and not the full offline Firefox setup for Windows. The VirusTotal scan having one result is likely a false positive. I am guessing it was about the 7zip (7zs.sfx) as some antivirus still trip up on that for the last couple decades even. Also this false positive tend to happen with the small online stub installer and not the full offline setup. Try the full setup for Windows from https://www.mozilla.orgfirefox/all/desktop-release/win64/ and pick your language. Since these files are compressed with 7zip you can extract the full setup and use it like it was a .zip version instead if you wanted to.