We are implementing a new extension API, called WebExtensions—largely compatible with the model used by Chrome and Opera—to make it easier to develop extensions across multiple browsers.
OK, what's the big deal?
A safer, faster, multi-process version of Firefox is coming soon with Electrolysis; we need developers to ensure their Firefox add-ons will be compatible with it.
This is a good thing.
To ensure third-party extensions provide customization without sacrificing security, performance or exposing users to malware, we will require all extensions to be validated and signed by Mozilla starting in Firefox 41, which will be released on September 22nd 2015.
You can disable this in about:config. Go ahead and try it in Nightly if you don't believe me (Set xpinstall.signatures.required to false)
We have decided on an approximate timeline for the deprecation of XPCOM- and XUL-based add-ons.
I have no idea what the fuck this even means.