Also, we remove the 'extensions.torbutton.restrict_thirdparty'
and use the 'privacy.thirdparty.isolate' pref. At the same time we
will need to add a checkbox to the about:preferences#privacy page
that toggles 'privacy.thirdparty.isolate'.

Note that 'privacy.thirdparty.isolate' will be renamed to
'privacy.firstparty.isolate' in Firefox 52.