The patch above fixes only the z-index issue, I'm not sure what to do with the blue-on-blue issue. Setting negative outline-offset wouldn't look good on selected/hovered tabs.
Bug 1692773 Comment 2 Edit History
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The patch above fixes only the z-index issue, I'm not sure what to do with the blue-on-blue issue. Setting negative outline-offset wouldn't look good on selected/hovered tabs. Maybe we'll deal with that once NNT is the default?