The old code would fail to detect the API doesn't exist and fall through to NS_OK: https://hg.mozilla.org/releases/mozilla-release/rev/74a972765968#l1.121 Now we fail with NS_ERROR_NOT_AVAILABLE. I'm going to guess the download code is not correctly handling that error?
Bug 1586228 Comment 8 Edit History
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The old code would fail to detect the API doesn't exist and fall through to NS_OK: https://hg.mozilla.org/releases/mozilla-release/rev/74a972765968#l1.121 Edit: Or actually, fail to initialize the service at all? Now we fail with NS_ERROR_NOT_AVAILABLE. I'm going to guess the download code is not correctly handling that error?