Trojan in firefox DownloadHelper extension

Hi everyone,

Today i opened up firefox to download a song on youtube by using DownloadHelper, which downloads videos in almost any format
The second i opened Firefox, this appeared

[b]*************************************
Threat Blocked !
Object has been placed in quarantine

Infection : Other:Malware-gen [Trj]
User :
Path :/Applications/net.downloadhelper.coapp.app/Contents/MacOS/bin/net.downloadhelper.coapp-mac-64

*************************************[/b]

I know that a few days ago i was redirected to a scam page copying Apple’s website (MacKepper scam). After doing a scan i saw that this page had successfully put the .pkg installation file for MacKepper in my computer without Avast blocking it :o

What could it be ? Did it infect Firefox’s files ?