In my patches for bug 1443578, I have to include an exception to ignore tab switch spinners caught by the flicker tests. This happens only on debug builds, mostly on the tabclose and tabstrip_overflow_underflow tests, and rarely on the tabswitch tests. https://treeherder.mozilla.org/#/jobs?repo=try&revision=845935ec303d29aef3fa79daab1ee3f58d7c01b6&filter-searchStr=debug shows the failures I had to whitelist. Marking as p5 because I only see this in debug builds.