=== Which add-ons will stop working after the deprecation? ===
Our goal is not to limit what add-ons can do in Firefox. The process we've started is designed to understand how we can extend Firefox in ways that are less brittle than the current XUL/XPCOM system. We will work with every developer who is interested to make their add-on work in Firefox. That said, add-ons may be lost if they are unmaintained or have maintainers who lack the time to port their add-on.
=== I'm writing a new extension today. What API should I use? ===