So I've been thinking about this rationally a lot. And basically... unless the actual light spectrum is harmful (blue/near UV) pixels are... JUST PIXELS. How does this hurt us?
Basically, in recent times subpixel rendering is all the rage. Flipping subpixels off and on to create a "smoother" image.
The answer is that I don't know what Firefox is doing, but I can make an educated guess that someone somewhere has started using libraries (MS, Firefox, etc.) that do "bad things with subpixels". Are they flickering? Maybe. Are they just hard to look at? Perhaps. Perhaps it's just an unnatural edge, similar to how bad anti-aliasing used to look like shimmering or flickering.
Your guess is nearly as good as mine. 😉