If you're on Nightly or e10s-multi is enabled by default with 4 content processes. Soon, Firefox 54 Aurora, will also have e10s is already -multi (4 processes) enabled by default! . A user-facing checkbox is available for controlling Electrolysis functionality. Open Preferences and check the "Enable multi-process" checkbox and then restart your browser:
[[File:E10s-toggle-in-preferences.png|324x150px|frameless|Nightly > Preferences > General > Enable multi-process]]