Part of the breakage is precisely due to the explicit intention to drop all the legacy APIs and the respective shims that have become too much of a burden to ma

Part of the breakage is precisely due to the explicit intention to drop all the legacy APIs and the respective shims that have become too much of a burden to maintain over time. One major technical change would be the deprecation plan of XUL/XBL (the toolkit used to build the interface, which could be freely accessed by addons) in favor of a wholly HTML future.