I have visually-induced dizziness/visual vertigo/"supermarket syndrome." Flashing and several common types of animation give me motion sickness and migraines. This includes animated gifs, animated pngs, blinking cursors, smooth scrolling, autoscroll, zooming, parallax, slide, carousel, marquee, jitter, judder, ease in-out, etc. as well as having non-scrolling or separately-scrolling elements alongside, in front of, or behind scrolling ones. So that's most app and web design right there... I find that opening about:config and setting layout.frame_rate 1 can help. But I have trouble typing with that setting, and if I want to watch safe video, can't do that without frame_rate 60. Intermediate rates such as 2 and 30 are too flashy. So an easy way to toggle this setting between 1 and 60 would be very helpful.