Mozilla announced the plans on their blog: We are announcing some major upcoming changes to Firefox add-ons. Our add-on ecosystem has evolved through incremental, organic growth over the years, but there are some modernizations to Firefox that require some foundational changes to support:

Taking advantage of new technologies like Electrolysis and Servo

Protecting users from spyware and adware

Shortening the time it takes to review add-ons

To help the add-on development community understand how we will enable these improvements, we are making four related announcements today:

We are implementing a new extension API, called WebExtensions—largely compatible with the model used by Chrome and Opera—to make it easier to develop extensions across multiple browsers.

A safer, faster, multi-process version of Firefox is coming soon with Electrolysis; we need developers to ensure their Firefox add-ons will be compatible with it.

To ensure third-party extensions provide customization without sacrificing security, performance or exposing users to malware, we will require all extensions to be validated and signed by Mozilla starting in Firefox 41, which will be released on September 22nd 2015.

We have decided on an approximate timeline for the deprecation of XPCOM- and XUL-based add-ons.

For our add-on development community, these changes will bring benefits, like greater cross-browser add-on compatibility, but will also require redevelopment of a number of existing add-ons. We’re making a big investment by expanding the team of engineers, add-on reviewers, and evangelists who work on add-ons and support the community that develops them. They will work with the community to improve and finalize the WebExtensions API, and will help developers of unsupported add-ons make the transition to newer APIs and multi-process support.

The strategy announced here necessarily involves a lot of trade-offs. Developers who already support Chrome extensions will benefit since they will have one codebase to support instead of two. Developers of Firefox-only add-ons will have to make changes. Those changes may require considerable development effort up-front, but we feel the end result will be worth that effort for both Firefox’s users and developers.

We want to reiterate our commitment to our add-on development community, and will work with you in porting extensions, designing new APIs, and creating innovative new add-ons that make Firefox great.

We will continue to post additional resources in the coming weeks and months to outline each of these changes in more detail, as well as provide support through our traditional channels via the Mozilla Developer Network, IRC (in #extdev), and the extension developer group.

Read the full announcement here.