How do I get the previous value?

I must be missing something here, because I'm invoking an attribute
modified event (using the DOMAttrModified event listener) by setting
the visibility of an object from hidden to visible
(object.style.visibility = "visible"). But I need to know, in the
handler, whether the handler was invoked by the visibility being
changed (or by some other event).

All of the DOM specs lead me to believe that the first parameter
passed to the handler routine has the answer, suggesting that
arguments [0].prevValue contains the previous value (ie:
visibility:hidden) and that arguments [0].newValue has the new,
updated value (ie: visibility:visible).

But I find that both have "visibility:visible" as their value. Is
there some black magic involved to get the previous value? I'm testing
this with Firefox 3.5.

Any help?
Anyone?
Please?

tia,
rts