This is not an error, but it may be useful for some users of the Mac version of Firefox to know. I have several tabs pinned to the left side of the tab bar (just the icons). Some of them I intend to use as "model" tabs to be duplicated for a session and the duplicate then deleted (such as bank logins). I have found that, if I left-click to select a pinned tab and THEN right click and select Duplicate Tab, the duplicate appears immediately to the right of the pinned tabs, and to the left of any open non-pinned tabs. But if I right-click the pinned tab WITHOUT first left-clicking it, it goes to the right of the LAST non-pinned tab on the right. This does not cause any errors, but it may be useful for some people to know; they can control where they want the duplicate tab to appear when they create it!