Why the Ajax calls could not be cached by Firefox?

Why the Ajax calls could not be cached by Firefox?

I noticed that all simple jQuery ajax calls are unable to cached by Firefox (no problem with other major browsers).

See the following test code:
$(document).ready(function(){
    $("#load").click(function(){$("#test").load("/test.txt");});
});

The mod_expires of Apache2 is enabled and test.txt can never be cached by Firefox via Ajax calls. (Firebug response status code 200 OK)