Finally managed to get it enabled on my system after a bit of frustration and nonsense.
First I set browser.tabs.remote.autostart to True
But then e10s was disabled because of add-ons, so I set extensions.e10sBlockedByAddons to False
Then it wouldn't enable because of "Accessibility tools". I don't use any accessibility tools.
I found in the Settings that the option to "show an on-screen keyboard when necessary" was enabled so I turned that off.
Still no e10s.
So I set browser.tabs.remote.autostart.disabled-because-using-a11y to False.
Still no e10s.
So I set accessibility.force_disabled to True.
Still no e10s.
Finally, after yet more Googling, I set accessibility.lastLoadDate to 0
Finally!
Apparently, if you intentionally or by accident, at some point in time, turned on accessibility, it bombs out.