You can install applications like SafeBrowser (Does not require root), however, this goes at the question as though you're going to abide by using this as your browser (you could enforce some application lock to ensure this happens, however).
Well, if you root your Samsung Galaxy T231 , through ES File Explorer you can edit your host file. This can be done quite simply, here is a link to the How To Geek website.
Another method as you said was to install Mozilla Firefox and run any one of the numerous add-ons to ensure you can block a website, however, against his relies on the user accessing only Firefox.
I'm quite sure with Tasker you can create a trigger to make changes to your Host file, or perhaps use IP Tables via AFWall+ to simply create application triggers and not website triggers.