Firefox Configs

DISABLE MIDDLE CLICK TAB BAR
Paste in profile/chrome/userChrome.css

/* TAB - Pointer Events [tested:68-73] */
#tabbrowser-tabs :-moz-any(.arrowscrollbox-scrollbox, .tabbrowser-arrowscrollbox) {pointer-events: none}
.tabbrowser-tab, #tabs-newtab-button, .tabs-newtab-button {pointer-events: auto}
.scrollbutton-up, .scrollbutton-down {pointer-events:auto !important}
.new-tab-popup, .new-tab-popup > * {pointer-events: auto}  /*container tabs - longpress*/

DISABLE WHATS NEW
in about:config set to false
browser.messaging-system.whatsNewPanel.enabled