If you receive an email in Thunderbird in OS X, there will be a notification window in the upper right corner of the screen. Here's a picture of this: http://lts.lehigh.edu/sites/lts.lehigh.edu/files/newmailprev.JPG You can click on this notification and then there will show up the email within a new window, but it's a simplified view and my add on does not work in it. The error console shows an error in the following function: function onPageLoad(event){ if(navigator.onLine){ if(gFolderDisplay.selectedCount>0){ var doc = event.originalTarget; // doc is document that triggered "onload" event var linkElement = doc.getElementsByTagName("a"); var linkNumber = linkElement.length; if(linkNumber > 0){ for(var i = 0; i<linkNumber;i++){ var aElement = linkElement[i]; -> var hrefValue = aElement.getAttribute("href"); The error message says, aElement is null. I think the problem is that Thunderbird can't read <a> elements on this special message window. In the "normal" view, my add on works just fine, the same with linux and windows.