You have a non-compositing window manager. This behavior always existed for the OpenGL compositor. Before bug 1574746, WebRender was forbidden for these menus (software rendering was enforced), now they use WebRender (OpenGL) as well.
Bug 1637244 Comment 3 Edit History
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You have a non-compositing window manager. This behavior always existed for the "OpenGL" compositor. Before bug 1574746, WebRender was forbidden for these menus (software rendering was enforced), now they use "WebRender" (the new OpenGL compositor) as well.
You have a non-compositing window manager. This behavior always existed for the "OpenGL" compositor. Before bug 1574746, WebRender was forbidden for these menus (software rendering was enforced), now they use "WebRender" (the new OpenGL compositor) as well. Addons already had this black border as they always used WebRender.