Firefox is planning to migrate users from extensions (add-ons) and themes to Jetpack and Personas respectively.
The Personas and Jetpack projects were experiments designed to make extending the platform easier, and dramatically reduce the overhead involved in version updates/UI changes. We knew from the beginning that we would have to trade off truly limitless customizations to produce a more stable API/pseudo-API, but we felt that was worth it to achieve our long-term goals for the project.
To compete and lead in the browser market now, we want and need to be able to move faster, and not have our hands as severely tied by add-on compatibility. We also want it to be safer to run with addons, and less disruptive to increasingly longer-lived and more complex browsing sessions. All of these requirements meant major technology changes, and some sacrifices. link.