I was under the impression that we added toolkit.cosmeticAnimations.enabled primarily to address "motion sickness" concerns. With that in mind toolkit.cosmeticAnimations.enabled 1) is probably badly named and 2) might currently controls animations it shouldn't control, but that seems orthogonal to this bug...
Bug 1478597 Comment 11 Edit History
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I was under the impression that we added toolkit.cosmeticAnimations.enabled primarily to address "motion sickness" concerns. With that in mind toolkit.cosmeticAnimations.enabled 1) is probably badly named and 2) might currently control animations it shouldn't control, but that seems orthogonal to this bug...