Poslední řešení z jejich fóra funguje, tohle přidat do userChrome.css.

/*** BEGIN Firefox 77 (June 2, 2020) Override URL bar enlargement ***/

  /* Compute new position, width, and padding */

  #urlbar[breakout][breakout-extend] {
    top: 5px !important;
    left: 0px !important;
    width: 100% !important;
    padding: 0px !important;
  }
  /* for alternate Density settings */
  [uidensity="compact"] #urlbar[breakout][breakout-extend] {
    top: 3px !important;
  }
  [uidensity="touch"] #urlbar[breakout][breakout-extend] {
    top: 4px !important;
  }

  /* Prevent shift of URL bar contents */

  #urlbar[breakout][breakout-extend] > #urlbar-input-container {
    height: var(--urlbar-height) !important;
    padding: 0 !important;
  }

  /* Do not animate */

  #urlbar[breakout][breakout-extend] > #urlbar-background {
    animation: none !important;;
  }

  /* Remove shadows */

  #urlbar[breakout][breakout-extend] > #urlbar-background {
    box-shadow: none !important;
  }