I'm not sure if this problem arose with the latest Firefox upgrade (I'm running MacOS 10.14.6 on an older MacbookPro) or has just appeared after a few days of use -- I think it's the latter. But I can no longer rearrange the sequence of tabs in a window, an absolutely essential feature. There's a contextual menu choice that gives me the option of moving a tab to the beginning, end or a new window, but that only helps in ~10% of situations. What gives??