As a decade+ Firefox user I think this is fabulous news, not because it'll be easier for Chrome exts to come to Firefox, but because the old extension 'API' was a heap of junk and made it trivial for third parties to really gum up Firefox internals. Also huge kudos to Mozilla for not bikeshedding some new/slightly incompatible API, which I'm not sure Google would have been capable of. :)

Chrome's API is much better designed, I suppose, because they had the first chance in 15 years to actually sit down and design a modern/safe API for this specific purpose. XPCOM just exposed JS to endless unsafe internal Firefox interfaces.