I can't find any official word that Firefox is supporting it. This article simply says it expects Firefox's will because of Firefox's reason for not supporting h.264 was that it wasn't open license.
Given how much caterwauling there's been about Firefox not supporting H.264 and the problems with Theora, I'd be very surprised if Firefox doesn't support VP8 soon after it's open-sourced. Too many people want it too badly for it not to happen.
