as discovered in bug 1083458 comment #4, web content is using the existence of expression closure for fingerprinting, and that would explain the high number in telemetry. now we can disable it on web content. the remaining concern is that disabling it might contribute to entropy for fingerprinting, in some case. In default configuration, platform.buildID would imply everything, but I guess there's some build that hides platform.buildID for privacy reason, maybe? if above might become some trouble, it's not necessary to hide it from web content separately from bug 1319512. otherwise, we can disable it on web content, prior to bug 1319512.