As of Firefox v54.0, multi-process support (also known as Electrolysis) is enabled by default in Firefox.
I had it working through various config tweaks for a few releases before that, but just noticed that at some point it had been disabled.
about:support reported it was disabled, with the issue:
0/1 (Disabled by accessibility tools)
This was a little weird as I didn’t recall having any accessibility tools enabled. I checked the Options and sure enough the option “Show a touch keyboard when necessary” (under Advanced->General) was enabled.
Disabling this option and restarting did not fix it, but then I found this page which carries this note:
accessibility.lastLoadDate– time of the last load. Electrolysis will be disabled for seven days post this time.
I edited this value in about:config – simply winding it back by some random amount (I used 1491307811 but basically anything more than seven days before whatever time it is should do the trick – and restarted Firefox. accessibility.lastLoadDate had vanished from about:config but multi-process mode was enabled again.