Firefox userChrome

userChrome.css

/* Remove circle and background and change padding, width and height from back button */
:root:not([uidensity="compact"]) #back-button > .toolbarbutton-icon {
  background-color: unset !important;
  border: unset !important;
  padding: var(--toolbarbutton-inner-padding) !important;
  border-radius: var(--toolbarbutton-border-radius) !important;
	width: calc(2 * var(--toolbarbutton-inner-padding) + 16px) !important;
	height: calc(2 * var(--toolbarbutton-inner-padding) + 16px) !important;
}

/* Fix Hover and Active states */
:root:not([uidensity="compact"]) #back-button:hover:not([disabled]) > .toolbarbutton-icon {
  background-color: var(--toolbarbutton-hover-background) !important;
  box-shadow: none !important;
}

:root:not([uidensity="compact"]) #back-button:active:not([disabled]) > .toolbarbutton-icon {
  background-color: var(--toolbarbutton-active-background) !important;
}

/* Fix padding */
:root:not([uidensity="compact"]) #back-button {
  padding-inline-start: 5px !important;
  padding-inline-end: 2px !important;
}

:root[uidensity="compact"] #back-button,
:root[uidensity="compact"] #PanelUI-menu-button {
  padding-inline-start: 2px !important;
}

/* Restore toolbar height */
:root:not([uidensity="compact"]) #nav-bar {
  height: 40px;
}

/* Add border when disabled */
/* :root:not([uidensity="compact"]) #forward-button[disabled="true"] > .toolbarbutton-icon, */
:root:not([uidensity="compact"]) #back-button[disabled="true"] > .toolbarbutton-icon {
  border: 1px solid var(--backbutton-border-color) !important;
}