XPCOM_DEBUG_BREAK appears to be set at https://searchfox.org/mozilla-central/rev/dc0adc07db3df9431a0876156f50c65d580010cb/taskcluster/taskgraph/transforms/job/mozharness_test.py#252
Bug 1525411 Comment 18 Edit History
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Trying to find the relevant place where XPCOM_DEBUG_BREAK is set to "stack-and-abort"...