If the response header "Last-Modified" is not available, the following code breaks Firefox 1.0:

var modRes = xml.getResponseHeader("Last-Modified");
if ( ifModified && modRes ) jQuery.lastModified[url] = modRes;

It throws the following exception:

Error: [Exception... "Component returned failure code: 0x80040111 (NS_ERROR_NOT_AVAILABLE) [nsIXMLHttpRequest.getResponseHeader]" nsresult: "0x80040111 (NS_ERROR_NOT_AVAILABLE)" location: "JS frame :: http://192.168.1.199/js/jquery.js :: anonymous :: line 5029" data: no] Source File: http://192.168.1.199/js/jquery.js Line: 5029

Fixed this with an try/catch block:

var modRes;
try {
    modRes = xml.getResponseHeader("Last-Modified");
} catch (e) {
    // do something or do nothing?
}
if ( ifModified && modRes ) jQuery.lastModified[url] = modRes;