You're supposed to enable menu-bar (https://support.mozilla.org/en-US/kb/restore-menu-bar-firefox#w_show-the-menu-bar) to have the orange button above the tabs, not behind them. This button was a very known (and great imo) feature in Firefox from v4.0 up to Firefox 28.0 (https://support.mozilla.org/en-US/questions/997275#answer-564181) and is to offer users to keep this design element included (even if not functional) in latest Firefox.