treechildren::-moz-tree-cell-text(container, closed, hasUnread, read) {
    text-decoration: none !important;
    font-weight:  bold !important;
    color: #000000 !important;
  }

  treechildren::-moz-tree-cell-text(container, closed, hasUnread, read, focus, selected),
  treechildren::-moz-tree-cell-text(container, hasUnread, read, focus, selected) {
    color: HighlightText !important;
  }

For some years I have a setup that makes a collapsed threads to be marked in bold when there is an unread message in it (instead of an underline). This stopped working in the latest 115 version. I did that based on this old thread https://support.mozilla.org/en-US/questions/1265223 To make this happen I had to: 1) Change the ..\Thunderbird\Profiles\name\chrome\userChrome.css to /* * Do not remove the @namespace line -- it's required for correct functioning */ @namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* set default namespace to XUL */ /* * Collapsed read thread, but the thread contains unread , set text to bold black */ treechildren::-moz-tree-cell-text(container, closed, hasUnread, read) { text-decoration: none !important; font-weight: bold !important; color: #000000 !important; } /* * If thread selected, revert to highlight text color */ treechildren::-moz-tree-cell-text(container, closed, hasUnread, read, focus, selected), treechildren::-moz-tree-cell-text(container, hasUnread, read, focus, selected) { color: HighlightText !important; } 2) settings -> General -> in the find box: Config editor: toolkit.legacyUserProfileCustomizations.stylesheets = true Sadly, this stopped working. Any idea how to make it work again?