with v1.2.3 + Mozilla Firefox 2.0.0.12,

this style of crossdomain $.getJSON() doesn't work,

since the callback function never gets called:

$.getJSON( 'http://jquery.com/api/', { callback: '?' }, function (){} );

although this style works fine:

$.getJSON( 'http://jquery.com/api/?callback=?', function (){} );

Here's a demo page:

http://iandeth.dyndns.org/mt/ian/archives/jquery-1.2.3-JSONP-bug/

the problem in the source code is with the regexp at line 2559:

jsre = /=\\?(&|$)/g

and line 2567:

s.data = jQuery.param(s.data);

and line 2580:

s.data.match(jsre)

where s.data ends up with some url encoded string like 'callback=%3F', which doesn't match the jsre regexp.

the old version (v1.2.1) of jsre variable used to have

the regexp with url encoded string:

jsre = /=(\\?|%3F)/g

so these places should be the placed to be fixed.

here's my version of a bug patch code:

http://iandeth.dyndns.org/mt/ian/archives/jquery-1.2.3-JSONP-bug/patch.txt

best,

toshi