Right-click the Desktop and select Properties, and open the Settings tab. You can adjust the screen resolution there.

You can paste this in the Code field in the Firefox > Web Development (or Tools) > Error Console and click the Evaluate button and enter 1024x768 on the prompt.

var Cc=Components.classes, Ci=Components.interfaces;

var ps = Cc["@mozilla.org/embedcomp/prompt-service;1"].getService(Ci.nsIPromptService);
var wm = Cc["@mozilla.org/appshell/window-mediator;1"].getService(Ci.nsIWindowMediator);
var nb = wm.getMostRecentWindow("navigator:browser");

var txt = 'width x height';
var obj = new Object; obj.value = nb.window.outerWidth + 'x' + nb.window.outerHeight;

if(ps.prompt(null, 'Resize', txt, obj, null, {}) && (obj.value != null)){
var vA = obj.value.split("x");
if(vA[0]) nb.window.outerWidth = vA[0];
if(vA[1]) nb.window.outerHeight = vA[1];
}