New Themes in Firefox - FAQ
For more information, see the "Improving Themes in Firefox" blog post
What’s going to happen to existing themes?
For Lightweight Themes (LWT), by far the most common type of theme in use, we will continue to seamlessly support them. There are thousands of themes that Firefox users love, and we don’t want to disrupt that. We will make some internal changes to addons.mozilla.org (AMO) and Firefox to support the new theme model, but expect these to be invisible to authors and users.
For Complete Themes, we intend to remove support for these themes in Firefox 57, due for release in November 2017. This is driven by the need to fix the problems mentioned in this post, to modernize our legacy XUL interface, and to bring major performance and UI improvements to Firefox users.
What does this mean for me as a Firefox user?
In the short term, nothing is changing. In the longer term, once this work is completed, the most popular types of themes will have greatly expanded theming capabilities, allowing for themes that look better and change more in Firefox. Users of Complete Themes will see them stop working later this year, reverting back to the default Firefox appearance.
All upside! Existing themes will continue to work seamlessly. We will be making some internal changes to how Firefox implements LWT support, to make use of this new architecture, but this should be invisible to you and your users. In the future we will likely be updating the AMO theme creation tool to allow using the new theming capabilities. The new theme model will be able to do everything LWTs can do, and much much more.
We expect to end support for Complete Themes in Firefox 57, due for release in November 2017. Some Complete Themes may find it feasible to use the “experimental” CSS support in the new theme system, and the ability to be layered on top of the default Firefox theme means avoiding compatibility headaches for the parts of the default theme you don’t want to change.
Does this mean users can install Chrome themes?
Not directly, we’re not holding our breath on the Chrome App Store supporting Firefox any time soon. The packaging format is also a bit different. But it should be possible for Chrome theme authors to make their themes available on Firefox, with almost no effort. We’re already seeing this happen with other Chrome add-ons thanks to the WebExtensions standard.