To enable or disable e10s, open Nightly's Preferences and check the "Enable E10S" checkbox. You will need to restart Nightly.
[[File:NightlyPreferencesE10s.png|648x333px324x150px|frameless|Nightly > Preferences > General > Enable E10s]]
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.