Firefox blue highlight color is sometimes darker. Why?

So I'm not sure if this is a Firefox bug/issue, or a Windows bug/issue, as I think Chrome does it too. On occasion when I start my computers (two of them), or do a reboot, the blue highlight color is sometimes darker. After a while it magically turns a lighter blue (as it did today (second screen clip)). The lighter blue is what I get 90% of the time especially if the computer has been always on for days. It's only a cold boot, or restart that I get the darker color. It never reverses (goes from light to dark). Does this happen to anyone else? I don't change themes, Windows colors, or extensions. It just seems random as sometimes a cold boot, or restart, it keeps the lighter blue.

Wow. I didn't know I'm the only one using Firefox. Or at least ever highlighted text. I guess I'm special.