Firefox Client Side Decorations: close, minimize, maximize on the left

userChrome.css

#TabsToolbar {
  direction: rtl;
}

#tabbrowser-tabs {
  direction: ltr;
}

.titlebar-buttonbox {
  display: flex;
}

.titlebar-min {
  order: 2;
}

.titlebar-max {
  order: 1;
}

.titlebar-close {
  order: 3;
  margin-right: 6px !important;
}

For Firefox 72 this shouldn't be needed anymore in Linux, where it initially was meant to be used, because Firefox now follows the relative system preference and moves the three buttons to the left if needed.