This issue also occurs on Windows with a High Contrast Black them enabled. I was able to reproduce this issue in Firefox 88, Beta 89.0b2 and Nightly 90.0a1 (2021-04-21).
Bug 1706290 Comment 2 Edit History
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This issue also occurs on Windows with a High Contrast Black theme enabled. I was able to reproduce this issue in Firefox 88, Beta 89.0b2 and Nightly 90.0a1 (2021-04-21).