This post is kept in English, because of some users in the Lighbox forum (Thread) having the same problem.
Using Lightbox 2.04 with area tags an rel="lightbox" or rel="lightbox[Group]" in Firefox will result in a java-script error.
Reason for the Bug:
rel is not an attribute of area neither in HTML 4 (see http://www.w3.org/TR/html4/index/attributes.html) nor in XHTML 1.0 Strict/Transitional (see http://www.w3.org/TR/xhtml1/#defs and take a look at the DTS..).
Consequence: imageLink.rel doesn't work with Firefox.
But Prototype is still able to get the attribute
So just replace imageLink.rel with Element.readAttribute($(imageLink), "rel") and enjoy Lightbox!