Make sure you do not drag a tab slightly down in the browser window while clicking the tab. 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 slightly down in the browser window. There is also a "Move Tab(s)" sub menu in the Tab bar context menu that works with multiple tabs selected. You can drag that tab back to the tab bar in the original window to undo this detaching if this happened by accident.

• Disable Tab Detach https://addons.mozilla.org/firefox/addon/disable-tab-detach/

This is an ongoing problem that is quite irritating. There needs to be either a way to disable it altogether or to set a delay or a greater distance that tabs need to be moved to go to a new window. It should not be possible to simply click on a tab with the intention of opening it and due to a slight tremor of the hand have it jump to a new window.