JS two if's statement

I wrote a javascript code which if you are browsing with Firefox and if the window is resized to pop up an alert box. However, my knowledge is not enough to see where is the mistake in the code I wrote. If someone can help me I will be really grateful. Thanks in advance.

$('FirefoxChecker').ready(function() {
    if (navigator.userAgent.indexOf("Firefox") > 0) && (window.onresize){
        alert("Some text here");
    };
});