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Some screens have dynamic brightness/contrast adjustment which can cause this effect.

Some monitors just have bad backlight. I had old one where maximizing terminal(which is mostly black) window caused taskbar and window handle also to turn almost black.

Are you sure it isn't some auto-adjustment on your monitor? Tried on Arch and Windows with Chrome and Firefox and no dimming.

Windows 10 and the same version of Firefox as you, and nothing gets dimmed on my screen.