I managed to reproduce the issue using an older version of Nightly on macOS 10.13. I retested everything on macOS 10.13, Ubuntu 18.04 and Windows 10 x64 using Firefox 84.0 and Nightly 85.0a1. The issue is not reproducing anymore.
Bug 1674121 Comment 5 Edit History
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I managed to reproduce the issue using an older version of Nightly on Windows 10 x64. I retested everything on macOS 10.13, Ubuntu 18.04 and Windows 10 x64 using Firefox 84.0 and Nightly 85.0a1. The issue is not reproducing anymore.