This article describes best practices when making extensions, including how to be kind to your users. It assumes that you are already familiar with Building an Extension.

User Interface

Tools menu items

Using the Tool menu option gives the author the maximum amount of choices. Whether the extensions should go at the top, bottom, or somewhere in between on the Tools menu, the author always has a choice. Ideally, the location would be below the Add-ons item, grouped with the other extension-related commands (menuitem:insertafter="javascriptConsole,devToolsSeparator"). Sub-menus should be used for single extensions needing multiple menu items, and a Tools menu item should not be created for options and preferences (for options and preferences, see the add-on manager). If possible, create a menu item in the menu where it is most applicable; for instance, a bookmark sharing extension should be called from the Bookmarks menu. To maintain the default theme, avoid the use of an icon next to the menu items.

Other UI elements

In general, toolbar items are very useful to end users because they can be removed or added to various toolbars as necessary. Status bar items should only be added for extensions that need constant monitoring, such as ad blocking, page ranking, or cookie management. Likewise, use context menu items sparingly — only for tasks that are done frequently or on specific elements of a web page.

Focus

Don't steal focus. It's not your extension's job to take focus from the web content. If the user loads a website and they ask for focus, they should get it. Overriding their request isn't very nice.

About dialogs

There is a default popup About dialog that is created from install.rdf data; creating a new XUL About box is usually unnecessary. You can decrease download size by omitting a customized About box. Make one only if you have a special feature that needs to be included — for example, a custom updater.

Theming

If you have XUL buttons in your extension that do functions similar to ones that already exist in a browser — for example, a feed reader that reloads and stops — use icons from the browser's theme. The icons makes the extension lighter, while providing more consistency, especially for users using different themes.

Extension icons

Unique icons are usually worth their download weight. They allow for easy identification among other extensions in the Extensions manager.

Coding practices

Namespace conflicts

There are many namespaces which extensions often must share with other consumers, be they other add-ons, web code, or the browser itself. These often include areas such as:

• Object prototypes, such as String.prototype, which are often extended to add methods to native objects.
• Global variables, such as top-level declarations on scripts loaded into shared windows or web pages.
• Expando properties of shared objects, such as document objects, or DOM nodes.
• IDs and class names in HTML and XUL documents, when extensions add elements to web pages or browser windows.
• chrome: or resource: packages, which are often defined in chrome.manifest files.
• about: page URLs.
• XPCOM contract IDs, which are often registered in chrome.manifest files.

While these are among the most common examples of namespaces in which conflicts can occur, there are many others. In general, care must be taken whenever defining a name anywhere that other code might do likewise. Strategies to avoid such conflicts include:

Avoid shared namespaces where possible

Many naming conflicts are best avoided by simply not sharing namespaces. Scripts can be loaded into their own globals, such as CommonJS modules, JavaScript modules, or Sandboxes, to avoid most global variable and prototype conflicts. Expando properties are best avoided using tools such as WeakMaps.

Prefix names in shared namespaces

When shared namespaces can't be avoided, the simplest solution to prevent conflicts is to use a distinct prefix for all of your names. Class names for HTML elements created by the Cool Beans extension, for instance, might all be prefixed with cool-beans-. Global variables might all be defined as properties of the CoolBeans object.

Some namespaces have specific conflict prevention conventions, which should be followed as appropriate. XPCOM contract IDs, for instance, should always begin with an @, followed by a domain name that the author controls, e.g., "@example.com/foo/bar;1"

It is important that the prefix that you use be unlikely to conflict with other code, and that it be indicative of the name of your add-on. Generic prefixes such as myextension-, or short prefixes such as ffx-, are likely to be used elsewhere, and therefore unsuitable to the purpose.

Call .noConflict(true) where applicable

Many common libraries which create global variables provide a method called noConflict, or similar, which revert any global variables they've declared, and return the object itself. For instance, calling jQuery.noConflict(true) will remove the window.jQuery and window.$ variables, and return the jQuery object itself, for future use by the caller.

When available, these methods should always be used to prevent conflicts with third-party code.

Names and Metadata

Naming

Be creative! Don't be redundant and include "extension," "Mozilla/Firefox/Thunderbird," or the version number in the name. Be original!

Descriptions

Use something that is descriptive, but that would fit in the default add-on manager width. The Mozilla extensions (Inspector/Reporter/Talkback) believe starting with a verb is the best way. For example, "Does an action in the browser."

Documentation

Assume that the vast majority of your users don't have inner working knowledge of Mozilla. Make sure your extension's homepage states the "obvious," including the purpose of your extension. Users also appreciate when your extension is shipped with a simple how-to document.

IDs

Firefox/Thunderbird 1.5 or later are much more strict about the IDs of extensions than their 1.0 counterparts. Make sure they're valid.

Version numbering

Please follow the Mozilla pattern: major version dot current incarnation dot security/bugfix release (like 1.0.7).

Internationalization

Locales

Always use locale DTDs and property files, even if providing your extension in one language. It will make translation of your extension to another language easier. It occurs more often than you would think.

Options

Firefox users like options. Lots of options. Try to include everything a user could ever want to customize in your extension, remembering more can be added later. For a large number of options for your extension, break the options window into multiple pages (tabs) that are well labeled. Don't hesitate to give long descriptions for each preference, as long as they are easy to understand, even for non-computer-savvy users. However, please make sure the default set of preferences is adequate — don't require people to tweak options in order to get your extension's core functionality.

Preferences' internal names

Internal Firefox preference names for extensions or to be clear, the name of the preference as it appears in the about:config, should start with "extensions.," then the name of the extension, with a dot, then the name of the preference. For instance, a boolean for the Reporter extension's option for hiding the privacy statement is "extensions.reporter.hidePrivacyStatement".

General

Dependencies

Requiring a user to download another extension in order to use yours isn't nice. Avoid the dependency on other extensions, especially extensions that you didn't develop.