OK, I can reproduce this by changing the Zoom to 90% or less by using Firefox API or `zoom` property. When I change the `zoom` property to 90%, the computed value of border width becomes 0.5px on Nightly, and the top and left border disappear.
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OK, I can reproduce this by changing the Zoom to 90% or less by using Firefox API or `zoom` property. When I change Zoom to 90% (by pressing View -> Zoom -> "Zoom out" in the top window menu, or changing Zoom to 90% in the top-right application menu), the computed value of border width becomes 0.5px on Nightly, and the top and left border disappear.