In Firefox, how do I disable "Install Missing Plugin" when my custom plugin is not found?

I don't want users to see the "install missing plugin" on their firefox when a custom plugin isn't found. How do I disable this?

<object height="0" width="0" id="mycustomPlugin" type="application/x-vnd-puppyt">开发者_JS百科</object>

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One way to do this -- there might be others -- is to check to see whether it's listed in the navigator.plugins array before creating the object tag. Something like this (some sample code cut-and-pasted from a recent project):

    function createVncServer(id) {
        console.log(navigator.userAgent);
        var vncDiv = document.getElementById("vncDiv");
        if (navigator.userAgent.indexOf("MSIE") > -1) {
            vncDiv.innerHTML = "<object id='" + id + "' classid='CLSID:42832F4C-3480-4450-A6B5-156B2EFC408F' codebase='http://localhost:51150/Resources/WinVncCtlInstaller.CAB' />";
        }
        else {
            // On Mozilla browsers, check to make sure the plugin is installed before we try to instantiate it, to avoid the weird "plugins required" message.
            var pluginInstalled = false;
            for (var i = 0; i < navigator.plugins.length; i++) {
                console.log(navigator.plugins[i].name);
                if (navigator.plugins[i].name == 'Alanta Remote Desktop Server') {
                    pluginInstalled = true;
                }
            }
            if (pluginInstalled) {
                vncDiv.innerHTML = "<object id='" + id + "' type='application/x-alanta-vnc' />";
            }
            else {
                promptForDownload();
                return null;
            }
        }
        var vncServer = document.getElementById(id);
        try {
            if (vncServer.Port > 0) {
                clearDownloadPrompt();
                return vncServer;
            }
            else {
                promptForDownload();
            }
        }
        catch (ex) {
            promptForDownload();
        }
    }