I can reproduce this issue on my machines Win 10 and MacOS11 on the latest Firefox Nightly 96.0a1 with Test Case from comment 18 and with TC: http://sandbox.pigsgrame.de/demos/bordercollapse/. I attached my results.
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I can reproduce this issue on my machines Win 10 and MacOS11 on the latest Firefox Nightly 96.0a1 with Test Case from comment 18 and with TC: http://sandbox.pigsgrame.de/demos/bordercollapse/. See my results attached.