userChrome.css file to move the Findbar up to the top right of Firefox, where it belongs.

userChrome.css

.browserContainer > findbar {
  -moz-box-ordinal-group: 0;
  position: fixed !important;
  right: 1em;
  border: 1px solid threedshadow !important;
  border-radius: 0 0 3px 3px;
  padding-left: 10px;

  /* Hide the "border" at the top by removing the box-shadow and background-image */
  border-top: none !important;
  box-shadow: none !important;
  background-image: none !important;

  /* Uncomment to add a transition from the top */
  /*
  transition: 400ms !important;
  z-index: 0 !important;
  */
}

.findbar-closebutton {
  margin-inline-start: 0.5em !important;
  vertical-align: middle !important;
  margin-bottom: 0.25em !important;
  height: inherit !important;
}