Out of browser, Theora probably does win; VP3 is computationally less complex than VP8, and there's no browser content or chrome to composit against, so it just blits to video memory.

At least in 10.4Fx, WebM has an accelerated pathway for decoding and rendering, but out of browser it would purely be a function of codec, and VP3 definitely wins.

Also, I should note that the library in Firefox 4/5 is different from the old one in 3.6, which uses the code you mention.