On recent UX builds the (Windows) application button (= Firefox button) can not be in titlebar anymore, if browser.tabs.drawInTitlebar is set to false. Most users won't notice this, because the option is set to true by default.
The upcoming add-on will support both browser.tabs.drawInTitlebar states true and false.
The corresponding setting still exists and can be accessed through about:config, but Mozilla will remove it one day. Thanks to Mike for providing a way to use this ui tweak even after its removal (in the future).