Vioxx versus Celebrex. what's the difference?

Because of two previous shoulder surgeries, my doctor started me on Vioxx. i had no ill effects from taking Vioxx and i am wondering why it was taken off the market. my doctor switched me to Celebrex. aren’t Vioxx and Celebrex identical in composition? why did the FDA take Vioxx off the market and not Celebrex? i am aware of the heart problems supposedly created by Vioxx. if both are the same, then why is Celebrex still out there?