>>907
以下の用に修正してください．
var tree = node.ownerDocument.createTreeWalker(node,NodeFilter.SHOW_ELEMENT,null,true);
↓
var tree = node.ownerDocument.createTreeWalker(node,chromeWindow.NodeFilter.SHOW_ELEMENT,null,true);

if(node instanceof HTMLAnchorElement && node.href){ gBrowser.addTab(node.href); ++opened; }
↓
if((node.nodeName == 'A' || node.nodeName == 'a') && node.href){ gBrowser.addTab(node.href); ++opened; }