Hiding and unhiding tabs

Firefox has started hiding all tabs in a window except the active tab. This has happened in two different windows. I have no idea how to hide tabs and don't want to. How can I unhide the tabs. When I try to do it one by one, the first tab rehides when the 2nd one appears. Can I unhide multiple tabs at once? Do I have to unhide them one by one, and if so how? How do I stop Firefox from hiding tabs?