Okay, this one worked, but it did get things a bit too tight. I'll try making some further adjustments.

cor-el said

If this still doesn't work then set a negative margin-top to move the items up. Add code to the userChrome.css file below the default @namespace line.

• http://kb.mozillazine.org/userChrome.css

---

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

:-moz-any(#bookmarks-view, #historyTree, #editBMPanel_folderTree, #placesList, #placeContent)
treechildren::-moz-tree-row {
  margin-top: -1px !important;
  min-height: 14px !important;
  height: 14px !important;
}