Firefox 77+ User Chrome

userChrome.css

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

/* Remove "Close" button on the tabs */
#tabbrowser-tabs .tabbrowser-tab .tab-close-button
{
    display: none !important;
}

/* Do not shift up when URL bar is active */
#urlbar[breakout][breakout-extend] {
    top: calc((var(--urlbar-toolbar-height) - var(--urlbar-height)) / 2) !important;
    left: 0px !important;
    width: 100% !important;
}

#urlbar[breakout][breakout-extend] > #urlbar-input-container {
    height: 100% !important;
    padding-block: 0px 2px !important;
    padding-inline: 0px 2px !important;
}