New Out Of Process Window in Firefox Nightly

In Firefox Nightly (and only in Firefox Nightly), there is new menu item called "New OOP window". It opens a new Firefox window that loads tabs in separate processes (electrolysis).

Bill McCloskey explains on bugzilla:

This idea started when we were talking about how to turn electrolysis on in nightly. Doing it in the near future would really anger a lot of people because basic stuff like printing still doesn't work. However, we do want to make it available to more people. A good middle ground would be to add a new menu option "New window in separate process" (name could be improved). It would be sort of like private browsing mode.