To workaround xml namespace selector problem. May I suggest to handle xml document selector separately and put the browsers specific code in it. Code goes like this(tested with FF2 and FF3 and IE6):

find: function(t, context){

....

if (jQuery.isXMLDoc(elem)){ handle xml document here

var quickNStag = new RegExp("^(#.?)([\\\\w_-]*):([\\\\w_-]*)|\\\\\\\\.");

re2 = quickNStag;

m = re2.exec(t);

m[0] = full name, m[1] = "." or "#", m[2] = namespace, m[3] = tagname

base on browser takes full name or just tag name

var tag = parseFloat(jQuery.browser.version) < 1.9 ? m[3] : m[0];

pass it to getElementsByTagName(tag)

...

}