Tab Buttons example for Firefox/SeaMonkey and Custom Buttons https://addons.mozilla.org/addon/custom-buttons/

tabButtons.js

// Tab Buttons example for Custom Buttons
// (code for "initialization" section)

// (c) Infocatcher 2013
// version 0.1.0

// Note: this is just an example, added buttons isn't accessible in SeaMonkey.

var tabButtons = {
	get btnPrototype() {
		var btn = document.createElement("toolbarbutton");
		btn.setAttribute("cb_id", "closeTab");
		btn.setAttribute("oncommand", '\
			for(var tab = this.parentNode; tab; tab = tab.parentNode) {\n\
				if(tab.classList.contains("tabbrowser-tab")) {\n\
					gBrowser.removeTab(tab);\n\
					break;\n\
				}\n\
			}'
		);
		btn.tooltipText = "Close Tab";
		delete this.btnPrototype;
		return this.btnPrototype = btn;
	},
	init: function() {
		addEventListener("TabOpen", this, true);
		addEventListener("drop", this, true); // For moved tabs in SeaMonkey
		addEventListener("mousedown", this, true); // To don't select tab after mousedown on button
		this.addStyles();
		this.addButtons();
	},
	destroy: function() {
		this.removeButtons();
		this.removeStyles();
	},
	handleEvent: function(e) {
		if(e.type == "TabOpen")
			this.addButton(e.originalTarget || e.target);
		else if(e.type == "drop") {
			for(
				var tab = e.dataTransfer.mozSourceNode;
				tab && "classList" in tab;
				tab = tab.parentNode
			) {
				if(tab.classList.contains("tabbrowser-tab")) {
					setTimeout(function() {
						this.addButton(tab);
					}.bind(this), 0);
					break;
				}
			}
		}
		else if(e.type == "mousedown") {
			var trg = e.originalTarget || e.target;
			if(trg.getAttribute("cb_id") == "closeTab") {
				e.preventDefault();
				e.stopPropagation();
			}
		}
	},
	styleNode: null,
	addStyles: function() {
		if(this.styleNode)
			return;
		// We can't use "style" or "class" attribute in Firefox...
		var style = '\
			@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");\n\
			toolbarbutton[cb_id="closeTab"] {\n\
				list-style-image: url("data:image/png;base64,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");\n\
				margin: 1px;\n\
				padding: 0;\n\
				border: none;\n\
				-moz-appearance: none;\n\
			}';
		this.styleNode = document.insertBefore(
			document.createProcessingInstruction(
				"xml-stylesheet",
				'href="' + "data:text/css,"
					+ encodeURIComponent(style) + '" type="text/css"'
			),
			document.firstChild
		);
	},
	removeStyles: function() {
		var sn = this.styleNode;
		sn && sn.parentNode.removeChild(sn);
	},
	get tabs() {
		var gBrowser = window.gBrowser || getBrowser();
		return gBrowser.tabs || gBrowser.tabContainer.childNodes;
	},
	addButtons: function() {
		Array.forEach(this.tabs, this.addButton, this);
	},
	removeButtons: function() {
		Array.forEach(this.tabs, function(tab) {
			var btn = this.getButton(tab);
			btn && btn.parentNode.removeChild(btn);
		}, this);
	},
	addButton: function(tab) {
		if(this.getButton(tab))
			return;
		var doc = tab.ownerDocument;
		var box = doc.getAnonymousElementByAttribute(tab, "class", "tab-middle box-inherit")
			|| doc.getAnonymousElementByAttribute(tab, "class", "tab-content")
			|| tab;
		if(!box)
			throw new Error("Can't find parent node for button");
		var btn = this.btnPrototype.cloneNode(true);
		box.appendChild(btn);
	},
	getButton: function(tab) {
		return tab.ownerDocument.getAnonymousElementByAttribute(tab, "cb_id", "closeTab");
	}
};

function destructor(reason) {
	if(reason == "update" || reason == "delete")
		tabButtons.destroy();
}
if(
	typeof addDestructor == "function" // Custom Buttons 0.0.5.6pre4+
	&& addDestructor != ("addDestructor" in window && window.addDestructor)
)
	addDestructor(destructor, this);
else
	this.onDestroy = destructor;

tabButtons.init();