Recently in my German thunderbird client one of the most important context menu entries for emails (e.g. in the list of emails) just vanished: The item was in German "in Themenbau anzeigen", which is probably something like "show in topic tree". Why is it gone? How can I get it back? I need the menu item all the time to understand the whole discussion of multiple emails belongin together (but I do NOT want to have a list of trees only).