Firefox’s redesign initially felt abrupt, but Pentadactyl polished away most of differing UI. However, somehow the spacing between the tabs and the main window got screwed up. The following Stylish workaround fixes the issue:

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

.tabs-newtab-button .toolbarbutton-icon {
  padding-top: 0;
  padding-bottom: 0;
}