Electrolysis has been enabled by default since Firefox 48. Source: https://wiki.mozilla.org/Electrolysis#Schedule.
This seems to be the case because I downloaded Firefox 53 for the first time on my new MacBook Pro and e10s wasn't enabled. I had to go to about:config and enable it manually.
Firefox 54 has e10s-multi, with multiple web content processes.
If you go to about:support in your Firefox 53, the "Multiprocess Windows" item in the first section ("Application Basics") should tell you why e10s is disabled for you, if it's disabled.
Edit: Ah, so this is weird. It looks like e10s isn't enabled when you first run Firefox. You need to quit, and then relaunch. On the second launch e10s is then enabled. This explains why I thought it was disabled by default (since it was my first time running Firefox on this machine).
[0] https://gist.github.com/dstaley/b17ce52947cf520ca793d545daee...
I can't tell from over here what preferences you changed and in what order and when. You said "I had to go to about:config and enable it manually"... did you restart after that? e10s pref changes require a restart to take effect. What was the state of the preferences before you did that, if you remember?
In any case, glad to hear it's working for you now.