We've had multiple instances where some performance tests fail on a specific branch, and cannot be reproduced anywhere else. They also cannot be reproduced locally.

Recently, we had mozilla-beta failures on Linux, and Mac that were unrelated to any changes in mozilla-central/mozilla-beta. It suggests an infrastructure failure, but there were no infrastructure changes when this started happening.

All of this suggests that there's either a pref that is interfering with something, or a pref setting is missing, and/or an external Firefox/Mozilla service is interfering here.