Disabling multi-process does not work

[Narretz]

The 1.0.1 cl says "disable multi-process firefox " in 9f28aa9.

However, I've noticed in FF53 and FF51 that about:config looks like this in FF:
browser.tabs.remote.autostart user-defined false
browser.tabs.remote.autostart.2 user-defined true

So the first one works, but the second one seems to get overwritten. Is it because Firefox itself sets this value somewhere?
I've tested with a clean FF51 installation.

I can also confirm that the second flag is definitely needed because after I manually set it to false, the behavior in my tests changes.

[mgol]

9f28aa9 doesn't say much about the reasons for disabling e10s; what are they? My worry is that most end users will run Firefox with e10s enabled so if the browser behavior differs in a Web-observable way, this may lead to broken sites.