So, I checked your demo in FF4 beta, Chrome, Safari and Opera on OSX, I can check that later in FF3.6 and others as well on Win7 later if you want to.
FF4.0 runs smoothly, but I had only 5 tabs opened. In Chrome the animation was a bit choppy with 12 tabs opened in background. Safari was clean of any tabs and animation went smoothly. In Opera it was choppy even though there were no tabs opened.
Firefox is pretty CPU hungry when you have lots of tabs opened (especially if you have some plugins active), that's why you may be experiencing slowdowns in animation, even Chrome didn't go smoothly when I had more tabs opened.
Let me know if you want me to test it under Win7 too.