I replied to the bug and asked for reporting to this BMO with a screenshot. My assumption: The user might have seen intermittent bug 1678804 (bug 1655924/bug 1635153 might be related). EGL reduces the chance for it to occur, but if one uses slow llvmpipe, it's perfectly reproducible with EGL as well: $ LIBGL_ALWAYS_SOFTWARE=1 MOZ_X11_EGL=1 firefox-esr
Bug 1695933 Comment 12 Edit History
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I replied to the bug and asked for reporting to BMO with a screenshot. My assumption: The user might have seen intermittent bug 1678804 (bug 1655924/bug 1635153 might be related). EGL reduces the chance for it to occur, but if one uses slow llvmpipe, it's perfectly reproducible with EGL as well: $ LIBGL_ALWAYS_SOFTWARE=1 MOZ_X11_EGL=1 firefox-esr