It seems to be a Firebug Bug
After reporting the mentioned bug to the Firefox guys they were not able to confirm it, and they are right!
It seems to be a Firebug bug!
Because if I do the same tests inline in the Firebug Lite console (not the Firefox extension in the browser!) everything works as expected.
>>> [] instanceof Array
true
I keep investigating
Pythoneer » Firefox3 beta3 has a strange Array behaviour said,
February 18, 2008 at 6:02 pm
[...] [Update] The bug seems to be in FireBug, not in Firefox [...]