These test failures on Mac look like they might be fairly involved to solve, so on mak's recommendation I've disabled the new tests on Mac and filed follow-up bug 1570474. A new revision with the tests disabled on Mac is now on Phabricator.
Bug 1565273 Comment 15 Edit History
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These test failures on Mac look like they might be fairly involved to solve, so on mak's recommendation I've disabled the new tests on Mac and filed follow-up bug 1570474. A new revision with the tests disabled on Mac is now on Phabricator. Here's try with `browser_urlbar_selection.js` re-run a number of times on Windows and Linux to check for intermittent failure: https://treeherder.mozilla.org/#/jobs?repo=try&revision=76e992ee84aa9e9d2412e8ebe67e8dda6b49e72f