I'm not sure it's possible yet to always browse privately on Firefox for Android, but you can delete cookies on the fly with ''Self-Destructing Cookies'' [https://addons.mozilla.org/en-us/firefox/addon/self-destructing-cookies/ https://addons.mozilla.org/en-us/firefox/addon/self-destructing-cookies/], one of my favorite addons. Also, if you go into private browsing mode (by hitting the tabs button in the top right corner and selecting the second tab ''private browsing'') you won't exit private browsing mode until you restart firefox (by swipping it away in an apps view, or restarting the phone, or one of those app killer apps).