If you're running the nightly channel, you already get a prompt introducing e10s and allowing you to enable it.

You can enable it in Aurora, but so far I've found way too much things to be broken for it to be useful.

Especially things like cookie-handling not working 100% and breaking login on lots of sites (like reddit, google, etc) makes enabling it a no-go, or at least did when I tried it a few weeks ago.

You do not want to enable e10s for Firefox Aurora, Beta, or release. The Nightly release channel has many critical bug fixes that are not in the other release channels. :)