Otherwise web pages don't render (although "local" chrome like the
preferences, about:* etc are rendered just fine in the same content
area). In the Unsafe Browser the failed pages are just completely
white; in the Tor Browser we get a "Gah. Your tab just crashed".

Note that without this setting our Tor Browser still gets e10s
disabled because of addons, namely our branding add-on which
apparently is not compatible.