Firefox 91 goddamned "floating tabs" fix

userChrome.css

/*
 * Based on https://www.askvg.com/tip-new-working-method-to-restore-classic-theme-and-ui-in-firefox-91-and-later-versions/
 * Modified a little for my own preferences
 */

.tab-background {
  /* I'm fine with rounded top corners */
  border-radius: 4px 4px 0px 0px !important;
  margin-bottom: 0px !important;
}

.tabbrowser-tab:not([selected=true]):not([multiselected=true]) .tab-background {
  background-color: color-mix(in srgb, currentColor 5%, transparent);
}

menupopup > menu, menupopup > menuitem {
  padding-block: 2px !important;
}

:root {
  --arrowpanel-menuitem-padding: 2px !important;
}

.titlebar-color {
  color: -moz-accent-color-foreground;
  background-color: -moz-accent-color;
}

#tabs-newtab-button {
  /* Height is not the same as tab height */
  /* Margin cannot be overriden, so I use transparent border */
  border-top: 4px solid #0000;
}