By "equivalency" I mean having access to equivalent APIs for specific functionality, as for example the inability to listen for window events that's breaking Vimperator and friends. (And electrovim! Have you noticed that it stops working on chrome pages that don't run scripts? Has the CSP bug been fixed? I didn't think it had been.)

Maybe it's not possible to support window event listeners without breaking e10s. But I see no a priori reason why it should be.

I haven't seen anyone in a serious discussion demanding 100% XUL API compatibility or feature coverage. (For the purposes of this comment, most discussion of the issue on HN is unserious.)