@anouk.lejeune.photography,

The regular malware removal specialists are not as familiar with Macs, but I’m going to try and find someone that can assist. Hang in there. In the meantime, turn off your iCloud from this machine, do not sync it with other devices, Turn off Time Machine (if you use it) and backing it up to an external HD (if you do), and do not use a USB stick with this machine.

Can you tell me what is happening in Firefox (FF) that is acting strangely? What else besides being redirected is strange with your machine? Can you correlate all this happening with something you did with your machine, like installing a new version of Flash or downloading something or going to a site on the web?

Is your machine up to date with the software patches including iTunes and the AppStore? What version do you have (click on the apple in the upper left corner > About this Mac)?

Have you contacted Apple regarding this problem yet?

Edit: I have asked the Avast Moderators familiar with Macs to assist.