https://blog.mozilla.org/addons/2017/04 ... ent-223852
https://blog.mozilla.org/addons/2017/02 ... ent-223780Starting with 57, many things will break that will also break add-ons. Most add-ons that aren’t WebExtensions will be broken regardless. Aligning with ESR is not a goal.
After 57 it will still be possible to run low-level code in Developer Edition and Nightly, via WebExtensions Experiments. It’s not quite an off switch like we have with signing, but it’s one way to run something like legacy add-ons. However, you should be aware that most legacy add-ons will break after 57, regardless. There are big changes coming to Firefox (some of which we aren’t talking that much about yet) that will break legacy add-ons anyway. 57 is not an arbitrary goal, but a fairly hard deadline.