Support for extensions using XUL/XPCOM or the Add-on SDK was removed in Firefox 57, released November 2017. As there is no supported version of Firefox enabling these technologies, this page will be removed by December 2020.
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.
Starting from Firefox 53, no new legacy add-ons will be accepted on addons.mozilla.org (AMO) for desktop Firefox and Firefox for Android.
Starting from Firefox 57, only extensions developed using WebExtensions APIs will be supported on Desktop Firefox and Firefox for Android.
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 use WebExtensions APIs if they can. See the "Compatibility Milestones" document for more information.
A wiki page containing resources, migration paths, office hours, and more, is available to help developers transition to the new technologies.
Firefox extension development is still an immature discipline, with a developer base consisting mostly of hobbyists and just a few organized development groups. There's a great deal of free online documentation available on XUL and extension development, but finding it and turning it into useful information can be a daunting task. Knowing where to look is critical and non trivial. This tutorial was aimed at compiling all the right resources for extension development and putting them in the right context, but there's much more to learn, and knowing how to find it is part of what we felt was necessary to teach.
Let's look into the resources that have helped us the most.
The Mozilla Developer Center
MDC should be your first stop when looking for documentation. If you find it lacking or missing some piece of information, please consider adding it once you've found it. Everyone looking for it later will thank you for it.
The AMO Developer Hub
The AMO Developer Hub is a great guide for add-on developers. It includes links to tutorials and documentation, development tools, and most notably, the Mozilla Add-ons Forum. This is a great place to ask questions and have them answered by experienced developers. It also includes discussions on add-on monetization, job postings and a long list of add-on ideas waiting to be developed.
The Mozilla Source
Sometimes you need more than just a tutorial. There are various tricks that you can't learn anywhere, but you know that Firefox implements them in some way. In these cases you should dive into Mozilla's source code and try to locate the code you need. One way to do this is to look into your Firefox installation directory, maybe unpack a few JARs and see what's inside. But this is very awkward and limited. It would be much better to be able to search and navigate through the huge code base with some ease.
Luckily, Mozilla provides exactly that in the Mozilla Cross-Reference. The main index in this page gives you options to look into the different product branches in development at Mozilla. Make sure you pick the one that matches your target versions.
All of these provide advanced search capabilities, including regular expression search and file path search. You'll be able to navigate the full source tree, inspect the change history for all files, and link to specific code lines. It takes a little getting used to, specially learning how to choose the best search queries, but it's an invaluable resource of information.
Several Mozilla community members maintain blogs that are updated frequently, often including information on API changes, bug fixes, useful tools and future release plans. Many of the tips and tricks included in this tutorial were discovered by reading these blogs.
There are several useful feeds that you can follow using a feed reader, such as Thunderbird. Here are some important feeds you should consider following:
- Planet Mozilla. This is the ultimate source for everything related to Mozilla. It brings together the feeds of most Mozilla employees, as well as several other members of the Mozilla community. Feeds are added frequently, so the information available through it will only grow with time. It can be hard to follow due of the sheer mass of information coming out of it (dozens of posts per day), but you'll certainly be up to date with pretty much everything if you take the time to read at least part of it.
- The about:mozilla newsletter is a weekly publication that includes the highlights of what is posted at Planet Mozilla. It is a good alternative if you feel Planet Mozilla is too much. You can also subscribe to receive it by email if you prefer that. Posts on about:mozilla also make it to Planet Mozilla.
- The Mozilla Add-ons Blog. This blog is maintained by the Add-ons Team at Mozilla, and it's a great way to stay up to date with AMO development and add-on review policies. This blog is also part of Planet Mozilla.
IRC and newsgroups
Mozilla developers use their IRC channels heavily. There are several help and development channels where you can discuss problems and other topics in real time with the people that have the answers. You'll need an IRC client in order to do this, and the Chatzilla extension works well for this purpose. Problems using IRC include: finding help when you have a big timezone difference with the United States, and no records of previously asked questions and their answers.
Another somewhat obscure communication channel is the Mozilla Newsgroups. They are also very diverse and active, and there's a good chance you'll get your queries answered. Thunderbird also comes in handy for reading and posting to the newsgroups. Another advantage is that discussions are archived and searchable through Google Groups.
And finally, a couple resources you should also keep in mind:
- Other extensions. There are thousands of extensions out there, and you may strike luck and find one that does exactly what you need. Be sure to look at the license the extension's code is released with before thinking about copying large sections of it. Also, remember to give credit when it's due.
- Mozilla's Bugzilla repository. Pretty much every code change done in Firefox and Mozilla projects is documented in this huge tracking database. But this also means that finding exactly what you need can be time-consuming, and often fruitless. The advanced search is very useful in filtering results, but it is our experience that it's often better to be too general than too specific. Better to scan a list of hundreds of bugs than getting no results at all, right?
- Anywhere else. Use your favorite search engine and patiently look for answers. It's better to take a long time finding an answer than taking even longer doing something that has already been done.
This tutorial was kindly donated to Mozilla by Appcoast.