userChrome to darken tooltips and context menu in FFx + get rid of white page on opening a tab, copy pasted and slightly modified from https://github.com/Izheil/Quantum-Nox-Firefox-Dark-Full-Theme

userChrome.css

/* DO NOT DELETE THIS LINE */
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

/* v Fix to make the pre-render loading page dark instead of white v */
#tabbrowser-tabpanels {background: #202020 !important}

tooltip, #statuspanel-label {
  -moz-appearance: none !important;
  background: rgba(60,60,60,0.9) !important;
  color: #eee !important;
  border: 1px solid #333 !important;
}

/* CONTEXT MENUS */

/* Main menu colors */
:root {
  --Context-menu-highlight: #25a;
  --Context-menu-background: rgba(68,68,68, 1);
  --panel-separator-color: rgba(0,0,0, .5) !important;
  --arrowpanel-background: #444 !important;
  --arrowpanel-color: #ccc !important;
  --in-content-box-background: #333 !important;
  --in-content-box-background-hover: #444 !important;
  --arrowpanel-dimmed: rgba(255, 255, 255, 0.1) !important;
  --arrowpanel-dimmed-further: rgba(255, 255, 255, 0.25) !important;
  --arrowpanel-dimmed-even-further: rgba(255, 255, 255, 0.4) !important}

/* Shortcut comments on main menu text color */
.subviewbutton[shortcut]::after, .subviewbutton[shortcut]::after, 
.PanelUI-subView .subviewbutton-nav::after, 
#appMenu-tp-category {color: #888 !important}

/* Zoom reset button on the main menu */
#appMenu-zoomReset-button {background-color: #666 !important}
#appMenu-zoomReset-button:hover {background-color: #333 !important}
#appMenu-zoomReset-button:active {background-color: #222 !important}

/* This one changes the letters color of all context menus */
menu, menuitem {
  color: #ccc !important;
  -moz-appearance: none !important}

/* Padding for all menu items (do NOT change unless you want an spaced or indented upon 
selection effect). The open in tabs bookmark item is added here since it also needs the 
padding that we remove in the next rule */ 
menuitem, .bookmark-item .openintabs-menuitem,
#history-menu menupopup > menu, #view-menu menupopup > menu {
  padding-top: 3px !important;
  padding-bottom: 2px !important}

/* This removes the unnecesary padding from the bookmark items */
.bookmark-item menuitem {
  padding-top: 0px !important;
  padding-bottom: 0px !important}

/* This changes the color on hover of the context-menu items */
menubar > menu[_moz-menuactive="true"],
menupopup > menu[_moz-menuactive="true"],
popup > menu[_moz-menuactive="true"],
menuitem[_moz-menuactive="true"]
{background-color: var(--Context-menu-highlight) !important}

/* This does the same but for disabled ones */
menubar > menu[_moz-menuactive="true"][disabled="true"],
menupopup > menu[_moz-menuactive="true"][disabled="true"],
popup > menu[_moz-menuactive="true"][disabled="true"],
menuitem[_moz-menuactive="true"][disabled="true"] 
{background-color: #333 !important} 

/* This changes the text color of the greyed out actions on the context menus */
menuitem[disabled="true"], popup > menu[disabled="true"],
menupopup > menu[disabled="true"], menubar > menu[disabled="true"]{color: #777 !important}

/* This one applies to all context menus (If you want to theme the browser this one is better instead 
of going one by one) */
menupopup:not(.PanelUI-subView) {background-color: var(--Context-menu-background) !important}

/* The following code changes the borders of all the context menus (except the ones above mentioned),
as well as ignoring the bookmark menu context menu */
menupopup:not(.PanelUI-subView) {
  -moz-appearance: none !important;
  border: 1px solid #111 !important}

/* Context menu separator color override */
menuseparator {
  -moz-appearance: none !important;
  margin: 1px 0 1px 2.3em !important;
  height: 1px !important;
  border: none !important;
  background: #222 !important;
  padding: 0 !important}

/* Context menu navigation tools (back, forward, reload) background color */
#context-navigation {
  -moz-appearance: none !important;
  background-color: #444 !important;
  padding-bottom: 0px !important}

/* This removes the borders for the bookmark menu button next to the url bar */
#BMB_bookmarksPopup, #BMB_bookmarksPopup menupopup {border: none !important}

/* This changes the text color of the hint text on the bookmarks popup menu */
.menu-accel-container .menu-iconic-accel {color: #808080 !important}
.menu-accel-container .menu-accel {color: #909090 !important}

/* Bookmark context menu > icon color and popup allow > icon color */
.bookmark-item .menu-right, menu .menu-right, 
#identity-popup-popup-menulist .menulist-dropmarker {filter: brightness(300%) !important}

/* Drop indicator that appears when moving a bookmark in a bookmark folder color */
.menupopup-drop-indicator {background-color: #999 !important}

/* Color of default icons on bookmark bar and bookmark popup (folder icons mainly) */
.bookmark-item > .toolbarbutton-icon[type="menu"], 
.bookmark-item[container="true"] .menu-iconic-icon {fill: var(--Bookmark-folder-icon) !important}

/* Color of default icons on bookmark bar and bookmark popup (world icon, aka not-favicon-found icon) */
.bookmark-item[scheme="http"], .bookmark-item[scheme="http"] .menu-iconic-icon {fill: var(--Bookmark-favicon-icon) !important}

/* Fix for the space above the "save as" option after the navigation buttons have been eliminated. Change 
if necessary depending on if you are using a custom OS theme (which may mess up the padding of the separators) */
#context-savepage {padding-top: 3px !important}