[jQuery] jquery ajax not working on firefox 2

Hi,
I'm using jquery ajax for a part of my site and it works on firefox 3,
ie 6, ie 7 and safari, but not firefox 2. Anyone come across this
before? Here's the ajax function
checkUsernameAvailability: function(i) {
//    var sBaseName = 'user_name';
    var sBaseName = i.split("reg_")[1];
    var sValue = $('#reg_'+sBaseName).val();
if (4 < sValue.length && sValue.length < 16) {
$.ajax({
         url: usernameAjax,
         type: 'POST',
            data: 'testUsername='+sValue,
         dataType: 'xml',
         error: function(){
//                alert('Error in checking username availability.');
                oRequired[i]= 0;
         },
         success: function(xml){
$(xml).find('exists').each(function(){
var exists = $(this).text();
if (exists == 'true') {
                        showHelp('error_user_name');
                        setTimeout('hideHelp(\'error_user_name\')',3000);
                        swapFlag(sBaseName,'nonvalid');
                        oRequired[i]=0;
                    } else {
                         checkAgainstBlacklist(i);
                    }
             });
         }
        });
    }
},
on firefox 2, it gets into error: function() and alerts me when that
alert line is uncommented.
any ideas?
Thanks