The latest update to the user interface sucks. The menu bar is gone, the message threading is confusing… It’s still the best desktop client but it’s not as good as it was before the “improved” UI.
As far as the menu bar is concerned, I find that I rarely go there as most things I need are located on the toolbar, which is customizable. For better or worse, they adopted the Gmail look and feel for conversations. If you don’t like the conversation mode, it can be turned off by going to menu/tools/settings/mail read and check “Disable conversations”.