No objection to FF itself!
I'd expect it be either 'Installing extra software' (who knows what anti-virus, toolbars or malware etc a youngster might install unwittingly) or it might get round parental controls.
Technically it is a criminal offence! Though I can't see any sensible parent going the nuclear route.
We had a work experience lad who downloaded on one of our PCs at least three applications without authorisation. He was also work shy and only lasted a couple of days.
Education and talking to your children is the key.
Though how non savy parents will cope should be a big concern to society along side radicalisation, grooming.... and even just the persuasive lad in the year above that asks for a nude selfie. Oh to be a parent of a teenager.