Add-ons using the techniques described in this document are considered a legacy technology in Firefox. Don't use these techniques to develop new add-ons. Use WebExtensions instead. If you maintain an add-on which uses the techniques described here, consider migrating it to use WebExtensions.
From Firefox 53 onwards, no new legacy add-ons will be accepted on addons.mozilla.org (AMO).
From Firefox 57 onwards, WebExtensions will be the only supported extension type, and Firefox will not load other types.
Even before Firefox 57, changes coming up in the Firefox platform will break many legacy extensions. These changes include multiprocess Firefox (e10s), sandboxing, and multiple content processes. Legacy extensions that are affected by these changes should migrate to WebExtensions if they can. See the "Compatibility Milestones" document for more.
A wiki page containing resources, migration paths, office hours, and more, is available to help developers transition to the new technologies.
The Mozilla Community is very rich and active. It's a very powerful tool you have when developing on top of the Mozilla platform, and you should take advantage of it in order to create a truly successful add-on .
AMO
AMO (Mozilla Add-ons) is the official Mozilla repository for add-ons. It allows you to upload, search and download all types of add-ons for Mozilla applications. This is the best way to get your extension to users from all around the world.
Adding your extension to AMO is easy. All you need is your XPI, a few screenshots (good ones will make your extension much more appealing), and a few minutes to fill a couple of forms. But this won't make your extension public right away. AMO has a Review Process that prevents malicious, insecure or low quality extensions to make it to the public site. Your extension begins in a sandbox with limited access, and once you nominate it an editor will review it and see if it is appropriate for the public AMO site, where everyone can see it and install it. The review process often takes a few weeks. Just be patient and make sure your extension follows AMO's policies when you nominate it. You can still get a good number of downloads while being in the sandbox.
One of the main advantages of using AMO is that it handles updates automatically for published add-ons. You just need to upload a new version of your extension, and once it's approved it will be pushed as an update to all your users. If you decide to host your own extension, you'll have to learn about the update system in order to push your own updates.
Babelzilla
Babelzilla is a worldwide community of localizers. Developers submit their extensions using the Web Translation System (WTS) and volunteers around the world translate them to different languages. The community is very active, and you can be sure to get translations for the most commonly used languages within a few days of submitting your extension. You'll also receive feedback that you wouldn't notice by testing only in one language, which can reveal other bugs in your locale handling.
There's a significant portion of users that use a localized version of Firefox, so you shouldn't neglect them. Using Babelzilla takes little time and is very valuable.
Mozdev
mozdev.org provides free project hosting and software development tools for Mozilla applications and add-ons. It is the only hosting service tailored to the needs of the Mozilla community.
It offers many necessary services such as bug tracking, source code repositories, download mirrors and many communication tools. There are other free hosting sites such as Souceforge and Google Code that are also very good, but not as specialized as Mozdev. You should pick the one that best fits your development needs.
Conclusion
That's it!
This is the end of the XUL School Tutorial. Hopefully this guide has helped you get started with add-on development and you're on your way to joining the large Mozilla add-ons developer community. The sheer volume of material can be overwhelming, but you can always come back and use this tutorial as a reference for your future development. Now all you need is a good idea (in case you didn't have one already) and get started.
Happy coding!
This tutorial was kindly donated to Mozilla by Appcoast.