Firebug

Firebug is a useful Firefox extension to debug JavaScript and more. To not raise error messages on browsers without Firebug, there’s a small script called firebugx.js, which creates empty
functions. Sascha Hameister has optimized this script a little bit and it now looks like this:

// FirebugX for browsers without Firebug
if (!window.console || !console.firebug) {
    var names = ["log", "debug", "info", "warn", "error", "assert", "dir", "dirxml", "trace",
                 "group", "groupCollapsed", "groupEnd", "time", "timeEnd", "profile", "profileEnd", "count"];

    var emptyFunction = function() {};

    window.console = {};
    for (var i = 0, count = names.length; i < count; ++i) {
        window.console[names[i]] = emptyFunction;
    }
}

You might also want to use Firebug Lite.