If your browser breaks in a way that you can't easily recover to change this setting, start Firefox in Safe Mode (by holding Alt/Option during start) which will force-disable e10s and allow you to enter the Preferences dialog and untick the checkbox.
== What to Expect ==
Generally the browser should be stable and usable, without major issues or crashes. Profiles that have lots of older addons will likely experience slowness or other issues. If you run into any trouble, start diagnosing by disabling addons and testing again.
==Schedule==