Firefox – Firefox extension that prevents websites from detecting what operating system you’re using

The (dead link) Anti-Disabler script on diveintogreasemonkey.org works if you replace document. with document.wrappedJSObject.

Link appears to be dead now, here's a wayback link: http://web.archive.org/web/20110830050224/http://diveintogreasemonkey.org/download/antidisabler.user.js

and here's the script:

// Anti-Disabler
// version 0.5 BETA!
// 2005-06-28
// Copyright (c) 2005, Mark Pilgrim
// Released under the GPL license
// http://www.gnu.org/copyleft/gpl.html
//
// --------------------------------------------------------------------
//
// This is a Greasemonkey user script.
//
// To install, you need Greasemonkey: http://greasemonkey.mozdev.org/
// Then restart Firefox and revisit this script.
// Under Tools, there will be a new menu item to "Install User Script".
// Accept the default configuration and install.
//
// To uninstall, go to Tools/Manage User Scripts,
// select "Anti-Disabler", and click Uninstall.
//
// --------------------------------------------------------------------
//
// ==UserScript==
// @name          Anti-Disabler
// @namespace     http://diveintomark.org/projects/greasemonkey/
// @description   restore context menus on sites that try to disable them
// @include       *
// @exclude       http://mail.google.com/*
// @exclude       https://mail.google.com/*
// ==/UserScript==

(function() {
    var e, i, all;

    document.onmouseup = null;
    document.onmousedown = null;
    document.oncontextmenu = null;

    all = document.getElementsByTagName("*");
    for (i = 0; i < all.length; i += 1) {
        e = all[i];
        e.onmouseup = null;
        e.onmousedown = null;
        e.oncontextmenu = null;
    }
})();

//
// ChangeLog
// 2005-06-28 - 0.5 - MAP - updated GMail URL
// 2005-04-21 - 0.4 - MAP - linted
// 2005-04-21 - 0.3 - MAP - exclude GMail
// 2005-04-18 - 0.2 - MAP - tidy code
// 2005-04-01 - 0.1 - MAP - initial release
//

You can use the Greasemonkey add-on to rewrite the alert function:

// ==UserScript==
// @name        Catch JS Alert
// @namespace   http://igalvez.net
// @include     http://*
// @version     1
// @grant       none
// @run-at      document-start
// ==/UserScript==

window.alert = function(message) {
    if(message == 'Adblock detected, please consider disabling it') {
        console.log(message);
    } else {
        confirm(message);
    }
}

The way this works is as follows:

If the alert box's message matches "Adblock detected, please consider disabling it", then discard it to the JS console (it will not be displayed). Otherwise, display the alert box as a confirm box.