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;
}

I fixed it for Firefox 145.0.1 - confirmed working

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

.tabbrowser-tab {
    position: relative !important;
    display: flex !important;
    align-items: center !important;
}

.tab-close-button {
    display: block !important;
    order: -1 !important;
    margin-inline: -15px 10px !important;
    opacity: 0 !important;
    flex-shrink: 0 !important;
    width: 16px !important;
    height: 16px !important;
    min-width: 16px !important;
    min-height: 16px !important;
    max-width: 16px !important;
    max-height: 16px !important;
    pointer-events: none !important;
    transition: none !important;
    padding: 0 !important;
}

.tab-close-button .toolbarbutton-icon {
    width: 16px !important;
    height: 16px !important;
    min-width: 16px !important;
    min-height: 16px !important;
    max-width: 16px !important;
    max-height: 16px !important;
    padding: 0 !important;
    margin: 0 !important;
}

.tabbrowser-tab[selected] .tab-close-button,
.tabbrowser-tab:hover .tab-close-button {
    opacity: 1 !important;
    pointer-events: auto !important;
}

.tab-content {
    margin-inline-start: 16px !important; 
    transition: none !important;
}

.tabbrowser-tab:hover .tab-content {
    margin-inline-start: 16px !important;
}

* {
    transition: background-color 0.15s ease-in-out, 
                color 0.15s ease-in-out, 
                border-color 0.15s ease-in-out;
}