Dev : Tips : Disable XUL cache

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You can add as many functions as you want. inject() evaluates all the functions in the file, but only executes the aFuncCall parameter. You can add as many functions as you want. inject() evaluates all the functions in the file, but only executes the aFuncCall parameter.

Revision as of 12:02, 19 January 2005

This page is part of the extension development documentation project.

Ask your questions in MozillaZine Forums. Also try browsing example code.

Note: development documentation is in process of being moved to Mozilla Development Center (MDC).



nglayout.debug.disable_xul_cache is a Mozilla preference useful for extension developers. Basically when it is set to false (which is default), Mozilla caches chrome XUL and JavaScript (and more) in a file named XUL.mfl or similarly. Therefore, when making changes to your extension, you have to restart Mozilla in order to get new version used. As it is very inconvenient, many extension developers set this pref to true.

Getting rid of JARs

However even when nglayout.debug.disable_xul_cache is set to true, Mozilla forbids rewriting *.jar files containing installed extensions. To overcome this and to avoid having to repackage every change you make, you can do one of the following:

  • Package your extension to use plain directories instead of a single JAR (before installing) - easiest.
  • Unarchive the JAR file you want to edit and make it permanent by updating chrome.rdf to point to the unpacked files instead of the JAR (after installing)
  • Use installed-chrome.txt to add your files to chrome, see [1] and [2] (not sure if it works in Firefox)

Changing install.rdf

Perhaps you would like to package your extension as a series of directories, rather than a jar file. This would allow, for example, an XPI that could be easily edited and perhaps checked back into a repository (e.g. a developer version).

Change the install.rdf from:

<Description about="urn:mozilla:extension:file:myextension.jar">


<Description about="urn:mozilla:extension:file:myextension">

An example of this technique is available. Also, myk is planning to update his tinderstatus extension to demonstrate this technique.

Editing chrome.rdf

We assume that the extension is installed in profile in this section.

Important!! Backup your profile before editing chrome.rdf!

  1. Unpack the JAR file you have, say [profile]\extensions\{YOUR-EXTENSION'S-GUID}\chrome\my-ext.jar to the same directory. You will get (at least) [profile]\extensions\{YOUR-EXTENSION'S-GUID}\chrome\content directory.
  2. In [profile]\chrome\chrome.rdf look for my-ext.jar! and delete it. For example replace this line

    with this:

    Note, that you should also delete jar: at the beginning of the path.

The overlay problem

There is still another issue to be solved: a Javascript that's called from the overlay. The steps above help you test scripts that are not part of the overlay, like your settings dialog. But they can't help you test the overlay scripts because Firefox caches those.

The solution is to have the overlay script load its code from an external file, which is reloaded and re-evaluated constantly. Here's an example:

var TheExt = {

	//aFuncCall is a string to be evaluated as a function call
	inject : function (aFuncCall) {
                var codeFileName =  "injected.js";

		// Get profile directory
		var file = Components.classes[";1"].
		get("ProfD", Components.interfaces.nsIFile);

		// Get the file with the code
		if (!file.exists()) {
			dump ("Error: " + file.path + "does not exist.\n");

		// Read the file into code
		var code = "";
		var fstream = Components.classes[";1"]
		var sstream = Components.classes[";1"]
		fstream.init(file, 1, 0, false);
		code +=;
		// Run the code
	doAction : function() {

dump ("Testing the extension.\n");

The file injected.js simply looks like this:

function doAction() {
	dump ("Action!\n")

You can add as many functions as you want. inject() evaluates all the functions in the file, but only executes the aFuncCall parameter.