Firefox has said they will implement V3, but "we will diverge from Chrome’s implementation where we think it matters and our values point to a different solution."
Specifically, "we have decided to implement DNR and continue maintaining support for blocking webRequest. Our initial goal for implementing DNR is to provide compatibility with Chrome so developers do not have to support multiple code bases if they do not want to. With both APIs supported in Firefox, developers can choose the approach that works best for them and their users."