Does this happen with one specific page/image, or everywhere? Could you take a screenshot comparing Firefox to another browser?

It happens on every website, is not a specific site problem, is a browser problem. I've tried this in multiple different machines at my office and is always the same issue. Colors on Quantum are off. I'm attaching a screenshot.

Huh, I can see in your screenshot that the colors are slightly different between the two using a color meter (although I have to admit that I can't tell the difference just by looking at them).

I'm not sure what's happening here. On my computer, if I take a photograph (like the copy of the iPhone photo from your screenshot that's from apple.com), I can see a difference between the pixels, but it seems like a positioning difference rather than a color one - one image is placed at a fraction of a pixel or something, which is causing the physical pixels on the screen to be rounded differently? However, in your screenshot, the solid color area at the top also has a slightly different color, so that wouldn't explain that. I've tried various images of solid colors, blocks of colors, etc. with a bunch of different color profiles, and they all seem to render the same between Firefox and Safari. Maybe someone else would have a better idea?