I changed the test so that it accepts more forms of the same message, so hopefully this doesn't break when the new error message is introduced. There is no change to functionality.
Bug 1611686 Comment 3 Edit History
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The new error message was added in bug #1259822. However we should be able to rely on the core part of the error message, as it shouldn't change too much due to webcompat issues. I changed the test so that it accepts more forms of the same message. There is no change to functionality.