Enable e10s in the Unsafe Browser (refs: #15023)

It now works just fine with Firefox ESR60.

config/chroot_local-includes/usr/share/tails/chroot-browsers/unsafe-browser/prefs.js

@@ -6,10 +6,6 @@ user_pref("network.proxy.socks_remote_dns", false);
 // the progress being made.
 user_pref("browser.download.panel.shown", true);
 
-// Web pages does not render when e10s is enabled, so we have to
-// disable it. Note that the "user_"-prefix is required.
-user_pref("browser.tabs.remote.autostart.2", false);
-
 // Disable searching from the URL bar. Mistyping e.g. the IP address
 // to your router or some LAN resource could leak to the default
 // search engine (this could include credentials, e.g. if something