Firefox Proton macOS Close Tab Left (updated for version 113)

userChrome.css

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); 

.tabbrowser-tab .tab-close-button {
  opacity: 0;
  margin-left: -4.5px !important;
  margin-right: 2px !important;
}

.tabbrowser-tab:not(:hover) .tab-close-button {
  display: none;
}

.tabbrowser-tab:not([pinned="true"]):hover .tab-close-button {
  opacity: 1;
  margin-inline-end: 0;
  -moz-box-ordinal-group: 0 !important;
  order: -1 !important;
  display: unset !important;
}

.tabbrowser-tab:not([pinned="true"]):hover .tab-icon-stack {
  -moz-box-ordinal-group: 99999 !important;
  order: 99999 !important;
}

.tabbrowser-tab:not([pinned="true"]):hover .tab-icon-stack:not([indicator-replaces-favicon]) {
  display: none;
}

Can you guys test this and confirm? I will then update the gist.

+1. Thank you savior.

This works until recent Firefox versions. In newer versions, sometimes when I point my mouse onto pinned tabs, the favicon becomes the close button, sometime not.