gecko-dev/browser/base/content/aboutSyncTabs.css

richlistitem[type="tab"] {
  -moz-binding: url(chrome://browser/content/aboutSyncTabs-bindings.xml#tab-listing);
}

richlistitem[type="client"] {
  -moz-binding: url(chrome://browser/content/aboutSyncTabs-bindings.xml#client-listing);
}