To summarize it:
In the address bar, write
about:config, answer yes to the warning, search for variables line after line, and set the values to those after equal sign:
Create a variable (or key for those who think of this as some kind of registry) with a boolean value (Right click in the pane with search results, Select "new"):
Restart the browser, then go to
about:support and as @gagarine mentioned, in Multiprocess Windows, there should be "1/1 (user activated)".
Now in Firefox 54, you should have Electrolysis on by default. You should check out the option dom.ipc.processCount and if it is still 1, turn it higher to match the physical core count, or if you know you use hyperthreading (most Core i7 and i3 on desktop, most Core i5 and i7 in laptops, AMD Ryzen), it is double that. (You can also see that in Task Manager under performance on Windows and in
cat /proc/cpuinfo | grep processor on Linux).
Read more about it here: https://medium.com/mozilla-tech/the-search-for-the-goldilocks-browser-and-why-firefox-may-be-just-right-for-you-1f520506aa35