Infected PC after program installation

I was downloading a Desktop app and, by accident, my PC got infected. I scanned my pc with no results, and the Firefox browser keeps opening and closing all the time. When I scan my pc with Bitdefender, it says that the computer is clean. How can I fix this?

Hello @Miles

Update your bitdefender and scan again, if still there is a problem, then you try with Malwarebytes. antivirus.Download Malware Removal 2023 | Free Antivirus Scan & Virus Protection Tool