Wow, that's terrible. How did you get to that page? If you terminate the app by double-pressing the home button and then swiping Firefox off the screen, is that same page restored when you start the browser up again? If it keeps coming back, you might try uninstalling Firefox. Long press its icon on the home screen until you get an icon to remove it. Hopefully when you reinstall it will be without the history of that page (although I haven't tried it myself).