Hello. Go to settings--sync--then tap the three bar icon at the top right of the screen, and press "remove account". As to keep your information private whenever someone is using your Firefox, press the three bar icon at the top right corner of the screen in Firefox. Then tap tools--new guest session. Now Firefox will be like it was just installed without any of your private information showing for others to see, for "guest browsing". To exit out of guest browsing, tap the three bar icon at the top right of the screen. Then at the bottom of the menu press "exit guest session" to go back to your normal Firefox profile. Hope this helps in some way!