A method for using Google Chrome-style locales in Firefox extensions.

chrome_locales_in_firefox.js

var MY_EXTENSION_STRINGS = {
	"strings" : {},

	get : function (key, substitutions) {
		if (key in this.strings) {
			var bundle = this.strings[key];

			var message = this.strings[key].message;

			if ("placeholders" in bundle) {
				for (var i in bundle.placeholders) {
					var regex = new RegExp("\\$" + i + "\\$", "g");
					message = message.replace(regex, bundle.placeholders[i].content);
				}
			}

			if (typeof substitutions != 'undefined') {
				if (typeof substitutions != 'object') {
					substitutions = [ substitutions ];
				}
			}

			if (substitutions) {
				for (var i = 0, _len = substitutions.length; i < _len; i++) {
					var regex = new RegExp("\\$" + (i+1), "g");
					message = message.replace(regex, substitutions[i]);
				}
			}

			return message;
		}

		return "";
	}
};

if ("@mozilla.org/extensions/manager;1" in Components.classes) {
	// Pre-Firefox 4
	(function (extension_id, string_object) {
		var extensionManager = Components.classes["@mozilla.org/extensions/manager;1"].getService(Components.interfaces.nsIExtensionManager);
		var installLocation = extensionManager.getInstallLocation(extension_id);

		try {
			var defaultLocale = JSON.parse(FileIO.read(installLocation.getItemFile(extension_id, "chrome/content/manifest.json"))).default_locale;
		} catch (e) {
			var defaultLocale = "en_US";
		}

		// Get the user's Firefox locale.
		var userLocale = Components.classes["@mozilla.org/preferences-service;1"].getService(Components.interfaces.nsIPrefService).getBranch("general.useragent.").getCharPref("locale");

		// Convert the userLocale to Chrome style.
		var userLocaleParts = userLocale.split("-");
		if (userLocaleParts.length > 1) userLocaleParts[1] = userLocaleParts[1].toUpperCase();
		userLocale = userLocaleParts.join("_");

		var localeOrder = [defaultLocale];

		if (userLocale != defaultLocale) {
			if ((underscoreIndex = userLocale.indexOf("_")) != -1) {
				localeOrder.push(userLocale.substr(0, underscoreIndex));
			}
			localeOrder.push(userLocale);
		}

		// Starting with the default, pull in all of the strings for each locale, 
		// overwriting previous ones if necessary.
		localeOrder.forEach(function (locale) {
			var messagesFile = installLocation.getItemFile(extension_id, "chrome/content/_locales/" + locale + "/messages.json");
			var messagesText = FileIO.read(messagesFile);

			if (messagesText) {
				var messages = {};

				try {
					messages = JSON.parse(messagesText);
				} catch (e) {
					// Invalid JSON.
				}

				for (var i in messages) {
					string_object[i] = messages[i];
				}
			}
		});
	})(MY_EXTENSION_ID, MY_EXTENSION_STRINGS.strings);
}
else {
	// Post-Firefox 4
	// Make sure that you've specified <em:unpack>true</em:unpack> in your install.rdf

	(function (extension_id, string_object) {
		var defaultLocale = "en_US";

		Components.utils["import"]("resource://gre/modules/AddonManager.jsm");  

		AddonManager.getAddonByID(extension_id, function (addon) {
			var addonLocation = addon.getResourceURI("").QueryInterface(Components.interfaces.nsIFileURL).file;

			// Get the user's Firefox locale.
			var userLocale = Components.classes["@mozilla.org/preferences-service;1"].getService(Components.interfaces.nsIPrefService).getBranch("general.useragent.").getCharPref("locale");

			// Convert the userLocale to Chrome style.
			var userLocaleParts = userLocale.split("-");
			if (userLocaleParts.length > 1) userLocaleParts[1] = userLocaleParts[1].toUpperCase();
			userLocale = userLocaleParts.join("_");

			var localeOrder = [defaultLocale];

			if (userLocale != defaultLocale) {
				if ((underscoreIndex = userLocale.indexOf("_")) != -1) {
					localeOrder.push(userLocale.substr(0, underscoreIndex));
				}
				localeOrder.push(userLocale);
			}

			// Starting with the default, pull in all of the strings for each locale, 
			// overwriting previous ones if necessary.
			localeOrder.forEach(function (locale) {
				var messagesFile = addon.getResourceURI("chrome/content/_locales/" + locale + "/messages.json").QueryInterface(Components.interfaces.nsIFileURL).file;

				var messagesText = FileIO.read(messagesFile, "utf-8");

				if (messagesText) {
					var messages = {};

					try {
						messages = JSON.parse(messagesText);
					} catch (e) {
						// Invalid JSON.
					}

					for (var i in messages) {
						string_object[i] = messages[i];
					}
				}
			});

			// Because this process is asynchronous in Firefox 4, you'll want to re-run
			// any localization scripts now that you run on document load, since the document may
			// have finished loading before this function ran, and if it did, all of your
			// locale strings would have been empty during the first call to your localizing
			// function.
		});
	})(MY_EXTENSION_ID, MY_EXTENSION_STRINGS.strings);
}