Mozilla Firefox 4.0 (beta) userChrome.css adjustments to place the close buttons on the left hand side of the tab.

userChrome.css

/*
 * 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 */


/* move favicon, throbber and text to the right so that the close button appears on the left */
.tabbrowser-tab .tab-icon-image {
  -moz-box-ordinal-group: 2 !important;
}

.tabbrowser-tab .tab-throbber {
  -moz-box-ordinal-group: 2 !important;
}

.tabbrowser-tab .tab-label {
  -moz-box-ordinal-group: 3 !important;
}

/* hide close button initially */
.tabbrowser-tab .tab-close-button {
  display: none !important;
}

/* show close + hide throbber & favicon on hover */
.tabbrowser-tab:hover .tab-close-button {
  display: block !important;
}

.tabbrowser-tab:hover .tab-icon-image {
  display: none !important;
}

.tabbrowser-tab:hover .tab-throbber {
  display: none !important;
}

/* app tabs should not behave like regular tabs, so let them stay normal (viz. don't show close on hover) */
.tabbrowser-tab[pinned]:hover .tab-close-button {
  display: none !important;
}

.tabbrowser-tab[pinned]:hover .tab-icon-image {
  display: block !important;
}