Notably, it'll remain disabled by default if you're using extensions (pretty much everybody here who is running Firefox?): The groups that will have to wait a bit for E10S account for about half of our release users and include Windows XP users, users with screen readers, RTL users, and the largest group, extension users.
You can manually enable it, though[0]: However, if you want to permanently opt-in, you can do so with a simple pref change. Just go to about:config and toggle browser.tabs.remote.autostart to true. On your next restart, e10s should be active. To verify that it is active, go to about:support and look for a number higher than 0 in "Multiprocess Windows".
I didn't realize they had this kind of fine-grained control over A/B-testing in their Firefox beta population, as Asa puts it: We have all the knobs. I understand where they are coming from, but it feels a bit weird to have a piece of software running on my computer checking in with a remote server to enable/disable functionality.