Blue flash completely destroyed, I confirm it.

This is my userChrome.css (I also have a megabar remover in there, that's not shown here):

* {

    transition-delay: 0ms !important;

    /* This is the magic to kill it 100% */
    transition-timing-function: steps(1, jump-start) !important;

    /* Animation Killer */
    animation-delay: 0ms !important;
    animation-duration: 0ms !important;
    animation-iteration-count: 1 !important;
    animation-timing-function: steps(1, jump-start) !important;

}

Also I have ui.PrefersReducedMotion=1 set in about:config.