Firefox for Windows whether as offline setup or online stub installer from www.mozilla.org/firefox/all and www.mozilla.org/firefox/new/ have never come with any form of malware. It would be an extremely hot topic here and elsewhere if your claim was indeed true. A small number of antivirus scanners tends to have false positives with Firefox things for so long or alarm people about malware in the Cache or tracking cookies. If you have the installer .exe you can upload it to https://www.virustotal.com/ Note a couple like clam on virustotal tend to have false positives when all others are clean. What hidden files? If it was in the Cache folder then you are not infected as they will either get overwritten over time as Cache gets used or deleted if you clear the Cache. This may be a older article but it still applies = [[Firefox cache file was infected with a virus]]