I was able to reproduce this issue under OS Windows 10 in Nightly Version 66.0a1 (2019-01-22) Build ID: 20190122215349. I was not able to reproduce anymore under Firefox release version 67 and Nightly versions 69.0a1
Bug 1521984 Comment 5 Edit History
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I tested this issue on Windows 10 with the Nightly build 20190122215349 before the fix and I was able to reproduce it. I also tested it using the latest Nightly build 20190523044159 and I’m not able to reproduce the issue. Based on this I will mark the bug as Verified.