I just found another piece to the puzzle. m-c:testing/mozharness/config/releases/bouncer_thunderbird.py needs updating as well, and I might as well move it into c-c with the rest of the mozharness configs while i'm at it.
Bug 1549748 Comment 3 Edit History
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I just found another piece to the puzzle. mozharness/releases/bouncer_thunderbird.py needs updating as well, but it doesn't exist yet. See bug 1550685.