No no no no no no! I'm an addon user and some of my addons heavily relies on this! If you remove this, some very important functionality will break! Don't remove it! If you do, I will immediately create a fork of Firefox with this feature enabled again, no matter how much effort it will take!

Please don't remove those, the reason I use firefox is extensions and the extensions I use require this to work.

If it's needed only for extensions, perhaps we could keep it for extensions. I understand we're not currently aiming for the same degree of interoperability for extensions as for the web as a whole. (Hopefully this will change as WebExtensions progress.) We shouldn't expose it to the web if it's not standard.