If you bring up DOM inspector then you will notice that the
'browsers/tabs' are actually located under the 'xul:panels' node which
has the anonid value of "panelcontainer" and not "tabcontainer". You
passed "tabcontainer" in to "GetAnonymousElementByAttribute" which is
not correct. You will also notice that this element contains multiple
'xul:notificationbox' elements (one for each tab) and that the
'xul:browser' node is the last child of the 'xul:notificationbox'.
The 'xul:browser" node is what you are looking for.