Fix default visibility of our options in about:prefs (again)

Changed again upstream (see 0d3242f), now for Firefox 32:
https://bugzilla.mozilla.org/show_bug.cgi?id=1017874

Fixing it prevents worsening a weird flickering at the first loading
of about:preferences, having E10s out-of-process tabs enabled
automatically (browser.tabs.remote.autostart).

And overlaying in-content/main.xul doesn't seem helpful.
RELEASE 0.8.1