Mozilla, the corporate backer of Persona, decided to end new feature development on it because of low adoption. Because of this, and very low usage numbers on our end, we decided to remove support.
While Firebase largely worked in PhoneGap prior to the latest releases, there were a few outstanding bugs specific to PhoneGap deployments that required special handling or attention, though websockets were not directly affected. As a sample, we had some previous errors around long-polling and creating iframes (used as a fallback when websockets fail) in PhoneGap, and most notably Firebase Simple Login was entirely unsupported in PhoneGap applications until this latest release.