It is a Firefox bug. More details here with the link to the bug report
https://github.com/NicoHood/arc-theme/issues/34
** Bug watch added: github.com/NicoHood/arc-theme/issues #34
https://github.com/NicoHood/arc-theme/issues/34
** Changed in: arc-theme (Ubuntu)
Status: New => Invalid
Attaching how Firefox 59 looks using Adwaita...
** Attachment added: "adwaita.png"
https://bugs.launchpad.net/ubuntu/+source/arc-theme/+bug/1745637/+attachment/5043564/+files/adwaita.png
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