This reproduces reliably for me on Ubuntu 20 with the Yaro system theme and firefox Light theme. As emilio pointed out, I needed to disable the system title bar. I.e. Customize toolbar, then uncheck "titlebar"
Bug 1718846 Comment 7 Edit History
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This reproduces reliably for me on Ubuntu 20 with the Yaru system theme and firefox Light theme. As emilio pointed out, I needed to disable the system title bar. I.e. Customize toolbar, then uncheck "titlebar"