We’re past API freeze so any API changes should be getting announced and communicated. In this case the change is tiny and unlikely to affect anyone. Have you heard of the interface amIWebInstallListener? If not then you can probably ignore this.
If you’re interested it is effectively what the add-ons manager backend code uses to communicate messages about webpage initiated add-on installations. An add-on or application might provide its own implementation if it wanted to provide its own UI for installs. Or an add-on might call it to get the normal UI to show up for some other cases but this is pretty rare.
The change is a method addition, onWebInstallDisabled is called when a webpage attempts to install an add-on but the preference xpinstall.enabled is false so installation is denied.