The issue is no longer reproducible on Nightly 92.0a1, 93.0a1, and Firefox 92 beta 1 - tested in strict mode and Private Browsing on macOS Big Sur 11.5, Windows 10, and Ubuntu 20.04.
Bug 1500076 Comment 8 Edit History
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The issue is no longer reproducible on Nightly 92.0a1, 93.0a1, and Firefox 92 beta 1 - tested in strict mode and Private Browsing on macOS Big Sur 11.5, Windows 10, and Ubuntu 20.04 x64.