Electrolysis "content" process sandbox already live on Windows
Mozilla started deploying Electrolysis over the past summer, starting with Firefox 48. Initially, multi-process support was rolled out for a small percentage of the Firefox userbase, who didn't use extensions. After a week, Electrolysis was then deployed to all users who used Firefox in a default state, without any extensions.
With Firefox 49, Electrolysis support was expanded to users that used a small number of well-tested extensions, and in Firefox 50, the current version, more Firefox users that used extensions received multi-process support.
Firefox 50, released at the start of the month, also included a first version of the Firefox sandbox feature, which now also covers the second Electrolysis process, the one that works with web content.
This Firefox web content sandbox feature is currently live only for Windows users, but Mac and Linux users are set to receive it as well "over the next few releases."