Undo After Playing Set

After I click play to see if I like what I see, I can't undo the thing I changed. I'll change something, play it, and want to undo it, and ctrl-z does nothing.
I use Mac and Firefox

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My browser version is:
Mozilla/5.0 (Macintosh; U; Intel Mac OS X; en-US; rv:1.8.1.16) Gecko/20080702 Firefox/2.0.0.16

Yeah, I have the same problem.

This is sort of by design, but I'll see what I can do to change it.