Enable e10s in the Tor Browser
All Browsers: disable e10s (multi-process Firefox). 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.
Attached are the the logs of when these failures occur.