Download with callback example for DragIt Firefox extension https://addons.mozilla.org/addon/dragit-formerly-drag-de-go/downloadSelectionWithCallback.js also works with Custom Buttons https://addons.mozilla.org/addon/custom-buttons/

downloadSelectionWithCallback.js

// Download images from selection example for Custom Buttons or DragIt Firefox extension
// https://addons.mozilla.org/addon/custom-buttons/
// https://addons.mozilla.org/addon/dragit-formerly-drag-de-go/

// Options:
var program = "%ProgramFiles%\\XnView\\xnview.exe";
var args = []; // Additional arguments, paths to temp files will be added after them
var tmpFilePrefix = this && this instanceof XULElement && this.localName == "toolbarbutton" // Looks like Custom Buttons
	? "cb_tmp"
	: "DragIt_tmp";

var selection = document.commandDispatcher.focusedWindow.getSelection();
var sourceNodes = [];
for(var i = 0, l = selection.rangeCount; i < l; ++i) {
	var range = selection.getRangeAt(i);
	var doc = range.startContainer.ownerDocument;
	var tmpNode = doc.createElementNS("http://www.w3.org/1999/xhtml", "div");
	tmpNode.appendChild(range.cloneContents());
	sourceNodes.push.apply(sourceNodes, tmpNode.getElementsByTagName("img"));
}

function openInProgram() {
	program = expandEnvironmentVariables(program);

	var exeFile = Components.classes["@mozilla.org/file/local;1"]
		.createInstance(Components.interfaces.nsILocalFile || Components.interfaces.nsIFile);
	exeFile.initWithPath(program);

	var process = Components.classes["@mozilla.org/process/util;1"]
		.createInstance(Components.interfaces.nsIProcess);
	process.init(exeFile);
	if("runw" in process) // Gecko 2.0+
		process.runw(false, args, args.length);
	else
		process.run(false, args, args.length);
}

function download(targetURL, sourceWindow) {
	var persist = Components.classes["@mozilla.org/embedding/browser/nsWebBrowserPersist;1"]
		.createInstance(Components.interfaces.nsIWebBrowserPersist);

	var fileName = targetURL.replace(/^.*\/|[?&#].*$/g, "");
	if(fileName.length > 30)
		fileName = fileName.substr(0, 15) + "_" + fileName.substr(-15);
	var tmpFileName = tmpFilePrefix + (fileName ? "_" + fileName : "");
	var tmpFile = Services.dirsvc.get("TmpD", Components.interfaces.nsIFile);
	tmpFile.append(tmpFileName);
	tmpFile.createUnique(tmpFile.NORMAL_FILE_TYPE, parseInt("0644", 8));
	Components.classes["@mozilla.org/uriloader/external-helper-app-service;1"]
		.getService(Components.interfaces.nsPIExternalAppLauncher)
		.deleteTemporaryFileOnExit(tmpFile);
	args.push(tmpFile.path);

	var targetURI = Services.io.newURI(targetURL, null, null);
	var privacyContext = "PrivateBrowsingUtils" in window
		? PrivateBrowsingUtils.privacyContextFromWindow(sourceWindow)
		: null;

	var wpl = Components.interfaces.nsIWebProgressListener;
	persist.progressListener = {
		onStateChange: function(webProgress, request, stateFlags, status) {
			if(!(stateFlags & wpl.STATE_STOP && stateFlags & wpl.STATE_IS_NETWORK))
				return;
			if(++done == count)
				openInProgram();
		},
		onLocationChange: function(webProgress, request, uri, flags) {
		},
		onProgressChange: function(webProgress, request, curProgress, maxProgress, curTotalProgress, maxTotalProgress) {
		},
		onSecurityChange: function(webProgress, request, state) {
		},
		onStatusChange: function(webProgress, request, status, message) {
		}
	};
	if(persist.saveURI.length <= 7)
		persist.saveURI(targetURI, null, null, null, null, tmpFile, privacyContext);
	else // Firefox 36+, see https://bugzilla.mozilla.org/show_bug.cgi?id=704320
		persist.saveURI(targetURI, null, null, null, null, null, tmpFile, privacyContext);
}

function expandEnvironmentVariables(str) {
	var env = Components.classes["@mozilla.org/process/environment;1"]
		.getService(Components.interfaces.nsIEnvironment);
	return str.replace(
		/%([^%]+)%/g,
		function(s, alias) {
			return env.exists(alias) ? env.get(alias) : s;
		}
	);
}

// Should be after declaration of all functions to work in DragIt
var count = sourceNodes.length;
var done = 0;
sourceNodes.forEach(function(sourceNode) {
	download(sourceNode.src, sourceNode.ownerDocument.defaultView);
});

downloadWithCallback.js

// Download with callback example for DragIt Firefox extension
// https://addons.mozilla.org/addon/dragit-formerly-drag-de-go/

// Options:
var program = "%ProgramFiles%\\XnView\\xnview.exe";
var args = ["%f"]; // %f will be replaced with path to temp file
var tmpFilePrefix = "DragIt_tmp";

// Get source node (supported only images and links),
// correct this to work not only with DragIt
var sourceNode = DragIt.dndParams.Source;

var targetURL = sourceNode.src || sourceNode.href;
var sourceWindow = sourceNode.ownerDocument.defaultView;

var persist = Components.classes["@mozilla.org/embedding/browser/nsWebBrowserPersist;1"]
	.createInstance(Components.interfaces.nsIWebBrowserPersist);

var fileName = targetURL.replace(/^.*\/|[?&#].*$/g, "");
if(fileName.length > 30)
	fileName = fileName.substr(0, 15) + "_" + fileName.substr(-15);
var tmpFileName = tmpFilePrefix + (fileName ? "_" + fileName : "");
var tmpFile = Services.dirsvc.get("TmpD", Components.interfaces.nsIFile);
tmpFile.append(tmpFileName);
tmpFile.createUnique(tmpFile.NORMAL_FILE_TYPE, parseInt("0644", 8));
Components.classes["@mozilla.org/uriloader/external-helper-app-service;1"]
	.getService(Components.interfaces.nsPIExternalAppLauncher)
	.deleteTemporaryFileOnExit(tmpFile);

var targetURI = Services.io.newURI(targetURL, null, null);
var privacyContext = "PrivateBrowsingUtils" in window
	? PrivateBrowsingUtils.privacyContextFromWindow(sourceWindow)
	: null;

function openInProgram() {
	program = expandEnvironmentVariables(program);
	args = args.map(function(arg) {
		return arg == "%f"
			? tmpFile.path
			: arg.replace(/%f/g, '"' + tmpFile.path + '"');
	});

	var exeFile = Components.classes["@mozilla.org/file/local;1"]
		.createInstance(Components.interfaces.nsILocalFile || Components.interfaces.nsIFile);
	exeFile.initWithPath(program);

	var process = Components.classes["@mozilla.org/process/util;1"]
		.createInstance(Components.interfaces.nsIProcess);
	process.init(exeFile);
	if("runw" in process) // Gecko 2.0+
		process.runw(false, args, args.length);
	else
		process.run(false, args, args.length);
}

var wpl = Components.interfaces.nsIWebProgressListener;
persist.progressListener = {
	onStateChange: function(webProgress, request, stateFlags, status) {
		if(!(stateFlags & wpl.STATE_STOP && stateFlags & wpl.STATE_IS_NETWORK))
			return;
		openInProgram();
	},
	onLocationChange: function(webProgress, request, uri, flags) {
	},
	onProgressChange: function(webProgress, request, curProgress, maxProgress, curTotalProgress, maxTotalProgress) {
	},
	onSecurityChange: function(webProgress, request, state) {
	},
	onStatusChange: function(webProgress, request, status, message) {
	}
};
if(persist.saveURI.length <= 7)
	persist.saveURI(targetURI, null, null, null, null, tmpFile, privacyContext);
else // Firefox 36+, see https://bugzilla.mozilla.org/show_bug.cgi?id=704320
	persist.saveURI(targetURI, null, null, null, null, null, tmpFile, privacyContext);


function expandEnvironmentVariables(str) {
	var env = Components.classes["@mozilla.org/process/environment;1"]
		.getService(Components.interfaces.nsIEnvironment);
	return str.replace(
		/%([^%]+)%/g,
		function(s, alias) {
			return env.exists(alias) ? env.get(alias) : s;
		}
	);
}