After branch merges (e.g. central -> aurora), the version is bumped: on aurora, n.0a1 becomes n.0a2, on central, it becomes n+1.0a1. Changesets after the version bump are correctly identified with their milestone, but anything that has landed on integration branches and wasn't merged to central by the time the merge happened are misidentified to the previous milestone. Technically, something similar happens on aurora for changesets between the last released aurora snapshot and the merge. Likewise on beta for changesets between the last released beta and the merge. I wrote on irc that a more accurate thing would be "what is the last FIREFOX_n_AURORA_BASE" tag that is an ancestor of that changeset?" but even that is not the whole story.
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