Track Mozilla's plan to implement MetricsDataPing

The metrics data folks at Mozilla plan to implement a mechanism to get a lot of data from Firefox users on an opt-out basis. See: https://bugzilla.mozilla.org/show_bug.cgi?id=718066 and https://wiki.mozilla.org/MetricsDataPing. Hopefully, there will be a pref to disable this. If not, I guess, the browser needs to get patched. This ticket is for the latter case in order to not get caught off guard.

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