Modify and extend Mozilla applications Add-ons add new functionality to Gecko-based applications such as Firefox, SeaMonkey and Thunderbird.
There are two main types of add-on: Extensions add new features to the application, while Themes modify the application's user interface. For both extensions and themes, Mozilla operates a repository at addons.mozilla.org, also known as AMO. When you submit add-ons to AMO they are reviewed, and after passing review they are made available to users. You don't have to submit add-ons to AMO, but if you do, users can take confidence in the fact that they have been reviewed, and you can benefit from AMO's visibility as a source for useful add-ons.
". Note: The extension created by this tutorial won't work in Firefox versions that don't have a static status bar (that is, Firefox 4 and up). You can find a more up-to-date tutorial in the XUL School tutorial The Essentials of an Extension. Since Firefox 4 (and other Mozilla 2 based applications) there are two types of extensions: Traditional, classic, or XUL extensions are more powerful, but more complicated to build and require a restart to install. This article explains how to build a traditional extension for Firefox. For a tutorial on building an extension for Thunderbird, see Building a Thunderbird extension. Quick Start A Hello World extension similar to what you can generate with the Extension Wizard is explained line-by-line in another tutorial from MozillaZine Knowledge Base. <? XUL Overlays.