rem: var webNavigation = getWebNavigation(); 
add: var webNavigation = gBrowser.webNavigation; 
context: function UpdateBackForwardButtons(){  var backBroadcaster = document.getElementById("Browser:Back");  var forwardBroadcaster = document.getElementById("Browser:Forward");    var webNavigation = getWebNavigation();  // Avoid setting attributes on broadcasters if the value hasn't changed!  // Remember, guys, setting attributes on elements is expensive!  They  // get inherited into anonymous content, broadcast to other widgets, etc.!  // Don't do it if the value hasn't changed! - dwh  var backDisabled = backBroadcaster.hasAttribute("disabled");  var forwardDisabled = forwardBroadcaster.hasAttribute("disabled");  if (backDisabled == webNavigation.canGoBack) {    if (backDisabled)      backBroadcaster.removeAttribute("disabled");    else      backBroadcaster.setAttribute("disabled", true);  }    if (forwardDisabled == webNavigation.canGoForward) {    if (forwardDisabled)      forwardBroadcaster.removeAttribute("disabled");    else      forwardBroadcaster.setAttribute("disabled", true);  }}
meta: 10234	/local1/tlutelli/issta_data/temp/all_js/javascript/2006_temp/2006/10234/b703a680af1d3650d75e6299dc72687209924f3d/browser.js/buggy/browser/base/content/browser.js