Although the Gecko platform supports multiple processes, the Firefox frontend is not designed to use them. Work to make the frontend (including addons) support multiple processes was begun in early 2013.
Enabling and Disabling Electrolysis
To enable or disable e10s, open Nightly’s Preferences and check the “Enable E10S” checkbox. You will need to restart Nightly.
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.