This roll-out won't necessarily be immediate, though. You can install Firefox 48 and, only some weeks later, get Electrolysis turned on retroactively. They are starting with about 1% of eligible users, which will ramp up to all eligible users over time or even be disabled if alarm bells start to ring.
Speaking of eligible users, there are quite a few conditions that will prevent you from getting Electrolysis. Namely, if you use extensions (it's unclear if they're talking about all extensions, or just ones that use certain APIs) then you will be kept on single-process. They don't specify why, but it could very well be the situation that I mentioned in the first paragraph.
Firefox 48 is scheduled to be released in six weeks (the first week of August).