Prettified version of https://github.com/PedroHLC/.dotfiles/blob/ssd-note/.mozilla/firefox/xfq51lm1.pedrohlc.way-1584130280343/chrome/userChrome.css

userChrome.css

/* ONE LINE TOOLBAR */
#navigator-toolbox {
  display: flex;
  flex-direction: row-reverse;
  justify-content: flex-end;
}

#titlebar {
  min-width: 40px !important;
  flex: 0;
  flex-basis: max-content;
}

#nav-bar {
  background-color: transparent !important;
  flex: 1;
  min-width: 500px !important;
  max-width: 640px !important;
}

#PersonalToolbar {
  flex: 0;
}

#tabbrowser-tabs {
  width: 100vw;
}

/* REORDER BUTTONS */
#nav-bar {
  display: flex;
}

#nav-bar-customization-target {
  order: 2;
  flex: 1;
}

/* REMOVE SEPARATOR FROM SANDWICH BUTTON */
#PanelUI-button {
  border: 0 !important;
  margin-left: 0 !important;
  padding-left: 0 !important;
}

/* TABS SAME SIZE AS TOOLBAR */
:root {
  --tab-min-height: 31px !important;
}

/* ONLY ICONS ON TABS */
.tab-label-container {
  display: none;
}

.tabbrowser-tab[fadein]:not([pinned]) {
  max-width: 34px !important;
  min-width: auto !important;
}

.tab-close-button {
  max-width: 0;
}

.tabbrowser-tab[visuallyselected="true"][fadein]:not([pinned]):hover {
  max-width: 60px !important;
}

.tabbrowser-tab[fadein]:not([pinned]):hover .tab-close-button {
  max-width: 24px !important;
}

/* HIDE NAV BUTTONS */
#back-button,
#forward-button {
  display: none;
}

/* That menu that appears with Alt, must not destroy the UI */
#toolbar-menubar:not([inactive="true"]) {
  margin-top: -29px;
  background: rgba(0, 0, 0, 0.5);
}

/* I'M WITH "SWAYWM", I DON'T NEED THESE */
#minimize-button,
#restore-button {
  display: none;
}