<profiledir>/preferences/extension-overrides.js will not be loaded in ESR 60

The code that read preferences from /preferences/extension-overrides.js was removed as part of https://bugzilla.mozilla.org/show_bug.cgi?id=1413413

We will have to do one of the following:

Restore the code that was removed (but see ticket:25750#comment:8).
Devise another way to handle the preferences that we include in extension-overrides.js

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