Base64 converter for Gecko-based browsers, accepts any valid URL string

base64.js

// https://gist.github.com/Infocatcher/2d001a8c72eb278c3686
// Based on code from Custom Buttons extension
// https://addons.mozilla.org/files/browse/261190/file/components/CustomButtonsService.js#L210

function ImageConverter(imageURL, feedback) {
	this.imageURL = imageURL;
	this.feedback = feedback;
	this.bytes = [];
	this.channel = Services.io.newChannel(imageURL, null, null);
	this.channel.notificationCallbacks = this;
	this.channel.asyncOpen(this, null);
}
ImageConverter.prototype = {
	// nsISupports
	QueryInterface: function(iid) {
		if(
			!iid.equals(Components.interfaces.nsISupports)
			&& !iid.equals(Components.interfaces.nsIInterfaceRequestor)
			&& !iid.equals(Components.interfaces.nsIRequestObserver)
			&& !iid.equals(Components.interfaces.nsIStreamListener)
			&& !iid.equals(Components.interfaces.nsIProgressEventSink)
		)
			throw Components.results.NS_ERROR_NO_INTERFACE;
		return this;
	},

	// nsIInterfaceRequestor
	getInterface: function(iid) {
		return this.QueryInterface(iid);
	},

	// nsIRequestObserver
	onStartRequest: function(aRequest, aContext) {
		this.stream = Components.classes["@mozilla.org/binaryinputstream;1"]
			.createInstance(Components.interfaces.nsIBinaryInputStream);
	},

	onStopRequest: function(aRequest, aContext, aStatusCode) {
		var contentType = this.channel.contentType;
		//var data = String.fromCharCode.apply(null, this.bytes);
		var data = this.bytes.map(function(code) {
			return String.fromCharCode(code);
		}).join("");
		this.channel = null;

		var dataURI = "data:" + contentType + ";base64," + btoa(data);
		this.feedback && this.feedback(dataURI);
	},

	// nsIStreamListener
	onDataAvailable: function(aRequest, aContext, aInputStream, aOffset, aCount) {
		this.stream.setInputStream(aInputStream);
		var chunk = this.stream.readByteArray(aCount);
		this.bytes = this.bytes.concat(chunk);
	},

	// nsIProgressEventSink
	onProgress: function(aRequest, aContext, progress, progressMax) {},
	onStatus: function(aRequest, aContext, status, statusArg) {}
};

// Usage example:
new ImageConverter("file://c:/autoexec.bat", function(dataURI) {
	gBrowser.selectedTab = gBrowser.addTab(dataURI);
});