Go to http://webstuff.nfshost.com/anim-timing/raftime.html and observe the "Callbacks: " value. Switch tabs away, wait a moment, then switch back. The callback count should not increase while the tab is in the background.
I'm duping this to 67873, which has details about this issue. It's not clear if background tabs should not fire or should fire at a reduced rate. We can hash it out on the other bug
*** This bug has been marked as a duplicate of bug 67873 ***