mozregression --launch 2020-08-20 --pref gfx.webrender.force-disabled:true -a https://wiki.mozilla.org/Firefox/Roadmap/Updates Confirmed. It also occurs with X11 and software rendering, so it's likely cross-platform. Only some horizontal lines are missing for me, but different ones than on your screenshot. I don't have a printer, but I assume the output would still be correct. (?)
Bug 1660474 Comment 3 Edit History
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mozregression --launch 2020-08-20 --pref gfx.webrender.force-disabled:true -a https://wiki.mozilla.org/Firefox/Roadmap/Updates Confirmed. It also occurs with X11 and software rendering, so it's likely cross-platform. Only some horizontal lines are missing for me, but different ones than on your screenshot. I don't have a printer, but I assume the output would still be correct. Edit: The PDF output looks correct.