The Marketplace is a store that features apps designed for any device that runs Firefox OS, Firefox for Android or Firefox desktop.
WhatsApp isn't directly available on Firefox OS, but you can still use a WhatsApp client on your device or chat with friends using Facebook or LINE.
This article explains how to clear the browsing history, the cookies and stored data on the Firefox Browser on Firefox OS.
Just press the power and home buttons at the same time. Your screenshot will be stored in the Gallery app.