@-moz-document url("chrome://browser/content/browser.xul") {

/* Remove Forward button when there's nothing to go Forward to */
#forward-button[disabled="true"] { display: none; }

/* Remove Back button when there's nothing to go Back to */
#back-button[disabled="true"] { display: none; }

/* Remove the drop-down when there's no history at all for the current tab */
#back-forward-dropmarker[disabled="true"] { display: none; }

}