I'm not usually a person to care about the version number, but this decision is awful. You don't make a change so radical to an established versioning scheme when the compatibility of addons is directly tied to the version number, the corporate users rely heavily on it for upgrade decisions and third parties rely on it for their distribution packaging policies. This is a complete utter mess.
It means that for quite a portion of time your browser won't work (the addons will be broken), it means that the Firefox derivatives packaged by some distributions will have unpatched security holes, it means that corporate users will have to keep a version that has unpatched security policies, because the new major version is not tested.
Even if they are making new versions with small incremental changes, at least support your older versions for some time, and make the changes necessary to not wreak havoc with your genius.