Answer:
the speed of light (all photons, naturally including those from the CMB, travel at the speed of light relative to you, regardless of frame or your velocity).
But I suspect you’ve asked this question because of a totally understandable but not quite correct set of assumptions about the universe. If my hunch is correct, you understand the CMB a bit like the skin of a balloon, gradually expanding over time. The Earth is a point somewhere in this balloon, and you’re asking about the Earth’s velocity relative to perhaps the center of this balloon. But that’s not quite how the universe works. It’s far more strange.
Our best hypothesis thus far is that the Big Bang happened from a single point. The “space” around us IS that single point. There is no origin point, since everywhere is by definition the origin point. You can see this for yourself by noticing that basically every star and galaxy we can see is - in the long run - moving away from us. And this is true regardless of whether you are on Earth or another star, or any point in the universe. So from this, it doesn’t really make sense to ask what the velocity of the earth is relative to the CMB rest frame, because space itself is expanding. Hopefully haven’t caused too many headaches!
Explanation:
