Multi-process Firefox (aka Electrolysis or just e10s) is till under development. If you want to test it, I recommend using Firefox Nightly (34) because it has recent e10s bug fixes and its File menu has a "Open e10s Window". You can test e10s and non-e10s windows without flipping an about:config pref or restarting Firefox. You can flip the optional pref browser.tabs.remote.autostart to true to enable e10s by default, which also enables extra "shims" for add-on compatibility so more add-ons will work than with "New e10s Window".
It looks like the prefs to enable it have been shipping beyond the nightly and are in the latest release.