javascript and XPath over Firefox

i have a page here :

<http://www.yvon-thoraval.com/DOM/xpath.xhtml>

which works with WebKit (Safari latest nightly) but only partly with
Firefox 2, the same with GranParadiso and Opera.

what means partly ?

if my xpath expression is "//body" the script returns nothing.

if my xpath expression is "//*[@attribute = 'some value']" the script
gives the correct answer...

in hope someone knows where is my mistake.

i should mention, as a first tentative i've used a script following
exactly a mozillation application note, afterwards i've followed some
advices from Opera's users, same result...

in the script itself i've left the various tentatives commented.
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