Make sure that you do not drag a tab slightly down in the browser window while clicking. Current Firefox versions have a feature called tear-off tabs.

You can detach a tab from the current window and open it in a new window by dragging a tab in the browser window. You can also do this via the right-click context menu of a tab: Move to New Window

You can drag that tab back to the tab bar in the original window to undo that detaching.

bug489729 (Disable detach and tear off tab):

• https://addons.mozilla.org/firefox/addon/bug489729-disable-detach-and-t

Thank You Very Much. Isn't there to disable this because it happens often and is very annoying. Also I can not drag it back.

You should be able to drag the tab back to its position on the Tab bar in the original window, either by dropping the tab on empty space on the Tab bar or between two tabs (you see a spacial up pointing arrow below the Tab bar).

You can install the extension I mentioned above to prevent this moving the tab to a new window.

Thank You. It's not the tabs that I am trying to get back but the windows while I clicked a tab it seems like I drag it down a little and it opened in a new window now I trying to get it back to the original windows but it does not drag back.