Firefox userChrome.css for a slightly more modern feel

userChrome.css

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
#nav-bar {
    padding: 0 .25em;
}

#back-button > .toolbarbutton-icon {
    border: 0 !important;
}

#permissions-granted-icon, 
#blocked-permissions-container {
    display: none;
}

#urlbar, 
#searchbar {
    border-color: white !important;
    transition: border-color 150ms ease-in-out;
}

#urlbar:hover, 
#searchbar:hover {
    border-color: hsla(240, 5%, 5%, .25) !important;
}

#urlbar[focused="true"], 
#searchbar[focused="true"] {
    border-color: Highlight !important;
}

toolbaritem, 
toolbarbutton {
    margin: 0 .25em;
    transition: background-color 150ms ease-in-out;
}

toolbaritem:first-child, 
toolbarbutton:first-child {
    margin-left: 0;
}

toolbarbutton.bookmark-item:not(.subviewbutton) {
    padding: 6px 6px !important;
}

toolbarbutton.bookmark-item:not(.subviewbutton):first-child {
    margin-left: 0 !important;
}

/* based on https://old.reddit.com/comments/fwhlva//fmolndz */
#urlbar[breakout][breakout-extend]:not([open]) {
    top: calc((var(--urlbar-toolbar-height) - var(--urlbar-height)) / 2) !important;
    left: 0 !important;
    width: 100% !important;
}

#urlbar[breakout][breakout-extend]:not([open]) > #urlbar-input-container {
    height: var(--urlbar-height) !important;
    padding-block: 0px !important;
    padding-inline: 0px !important;
}

#urlbar[breakout][breakout-extend][breakout-extend-animate] > #urlbar-background {
    animation-name: none !important;
}

#urlbar[breakout][breakout-extend]:not([open]) > #urlbar-background {
    box-shadow: none !important;
}