This bug 1562839 says how to enable the feature only in Nightly Changesets: https://hg.mozilla.org/mozilla-central/rev/5289c222f0d0 Honza
Bug 1564105 Comment 1 Edit History
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This bug 1562839 can be used as an example of how to enable a feature only in Nightly Changesets: https://hg.mozilla.org/mozilla-central/rev/5289c222f0d0 Honza