(In reply to User Dderss from comment #1) Ran mozregression with an older copy of my main profile and was asked to start in safe mode: Attached screencast looks simlar to comment 1, but it's bug 1489463 with a different appearance.
Bug 1567583 Comment 2 Edit History
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(In reply to User Dderss from comment #1) Ran mozregression with an older copy of my main profile and was asked to start in safe mode: Attached screencast looks simlar to comment 1, but it's bug 1489463 with a different appearance. (Just in case of doubt: My also running main Nightly has a different theme. It's really a corruption of this window.)