You've probably encountered sites that open up new windows without menu bars or toolbars and so on. With this little Mozilla trick, you can press Control+F5 to get your chrome back. Or, press Shift+Control+F5 to get back the chrome in all browser windows, which is useful if someone has disabled the keyboard in a window. First, add the following to userChrome.css in your profile's chrome directory:

tabbrowser {
  -moz-binding: url('file:///stuff/rechrome.xml#tabbrowser');
}

You may use a different path if you wish. Here, /stuff/rechrome.xml is used. Then add the following to that file:

<?xml version="1.0"?>
<bindings id="rechromeBindings"
   xmlns="http://www.mozilla.org/xbl"
   xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
  <binding id="tabbrowser" extends="chrome://global/content/bindings/tabbrowser.xml#tabbrowser">
    <handlers>
      <handler event="keypress" keycode="VK_F5" modifiers="control">
        window.top.document.documentElement.setAttribute("chromehidden","");
      </handler>
      <handler event="keypress" keycode="VK_F5" modifiers="shift,control">
        var wm=Components.classes["@mozilla.org/appshell/window-mediator;1"].getService();
        wm=wm.QueryInterface(Components.interfaces.nsIWindowMediator);
        var wlist=wm.getEnumerator(null);
        while (wlist.hasMoreElements("navigator:browser")){
          var docelem=wlist.getNext().document.documentElement;
          if (docelem.hasAttribute("chromehidden"))
            docelem.removeAttribute("chromehidden");
        }
      </handler>
    </handlers>
  </binding>
</bindings>

Feel free to change the keys to something else. Not that this trick may require a fairly new version of Mozilla. Slight changes may be needed for older versions or for Phoenix.