Thank you for your question. If I may ask, what version is your Firefox OS device? You have to log into youtube with a google account and change the settings for the browser. It is called Safety Mode. Tap this and go to Parental Controls. Turn on YouTube Safe Search. Then click on 'Save and lock Safety Mode on this browser'. You will need the credentials of the google account that set the controls as far as I understand.

What were the results?