Make Firefox Quantum look native on macOS

userChrome.css

@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);
/* Moves tab close button to left of tab */

@-moz-document url(chrome://browser/content/browser.xul) {
  #TabsToolbar #tabbrowser-tabs .tabbrowser-tab:not([pinned]) .tab-close-button {
    -moz-box-ordinal-group: 0 !important;
    -moz-margin-start: -4px !important;
    -moz-margin-end: 4px !important;
  }
}

.tabbrowser-tab:not([selected]) .tab-close-button {
  visibility: hidden !important
}

.tabbrowser-tab:not([selected]):hover .tab-close-button {
  visibility: visible !important
}

/* Tab bar below Navigation & Bookmarks Toolbars */

#nav-bar {
  /* main toolbar */
  -moz-box-ordinal-group: 1 !important;
  box-shadow: none !important;
}

#PersonalToolbar {
  /* bookmarks toolbar */
  -moz-box-ordinal-group: 2 !important;
}

#TabsToolbar {
  /* tab bar */
  -moz-box-ordinal-group: 3 !important;
  padding-top: 0 !important;
}

/* Clean up spacing */

.titlebar-placeholder {
  display: none !important;
}

toolbarbutton.bookmark-item {
  padding-top: 2px !important;
  padding-bottom: 2px !important;
}

/* Vertical space for min/max/close buttons above the main toolbar */

/* On Mac, if not showing title bar, allow 32 pixels */

#main-window[tabsintitlebar="true"]:not([inFullscreen="true"]) #toolbar-menubar:not([accesskey])~#nav-bar {
  margin-top: 32px !important;
}

/* On Windows, if not showing title bar or menu bar, allow 22 pixels */

#main-window[tabsintitlebar="true"] #toolbar-menubar[autohide="true"][inactive="true"]~#nav-bar {
  margin-top: 22px !important;
  /* More px may be better on Win10 */
}

/* Sorry, Linux users, I don't know the best approach for you! */

/* Background for Light and Dark themes */

#main-window[lwthemetextcolor="bright"] #TabsToolbar, #main-window[lwthemetextcolor="dark"] #TabsToolbar {
  background-color: var(--chrome-secondary-background-color) !important;
  background-image: none !important;
}

#main-window[lwthemetextcolor="dark"] .scrollbutton-up, #main-window[lwthemetextcolor="dark"] .scrollbutton-down, #main-window[lwthemetextcolor="dark"] .tabs-newtab-button, #main-window[lwthemetextcolor="dark"] #new-tab-button, #main-window[lwthemetextcolor="dark"] #alltabs-button {
  fill: var(--lwt-text-color) !important;
}

/* Left and right borders on Win 7 & 8, but not on 10 and later: */

@media (-moz-os-version: windows-win7), (-moz-os-version: windows-win8) {
  /* Vertical toolbar border */
  #main-window[sizemode=normal] #navigator-toolbox>toolbar#TabsToolbar {
    border-left: 1px solid hsla(240, 5%, 5%, 0.3) !important;
    ;
    border-right: 1px solid hsla(240, 5%, 5%, 0.3) !important;
    ;
    background-clip: padding-box;
  }
}

/* Override vertical shifts when moving a tab (9 Jan 2018) */

#TabsToolbar[movingtab] {
  padding-bottom: 0 !important;
}

#TabsToolbar[movingtab]>.tabbrowser-tabs {
  padding-bottom: 0 !important;
  margin-bottom: 0 !important;
}

#TabsToolbar[movingtab]+#nav-bar {
  margin-top: 0 !important;
}

/* Fix tab bar height */

#tabbrowser-tabs {
  --tab-min-height: 18px !important;
  margin-left: 0px !important;
  height: 25px !important;
}

/* Hide blue stripe on active tab */

.tab-line[selected="true"] {
  opacity: 0 !important;
}

Works for version 64.0.

Copy to $HOME/Library/Application Support/Firefox/Profiles/xxxx.default/chrome.