Comment 2 for bug 1918249

OK, re-enabling userChrome.css according to

http://forums.mozillazine.org/viewtopic.php?f=30&t=3064381&start=6

and creating a userChrome.css as follows:

/* Thunderbird userChrome.css */

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

#folderTree > treechildren::-moz-tree-image {
  width: 10px !important;
  height: 10px !important;
}

#folderTree > treechildren::-moz-tree-cell-text {
  padding-bottom: 1px !important;
}

provides a temporary solution!