The big problem is that this happens from time to time without intention on my part. One way to activate tabs is to left-click on them. If that button is pressed to long and the mouse moved downwards just a bit it will move the website in the tab to the new window.

There is apparently no way of disabling the feature to drag and drop tabs into new windows in Firefox at this point. It happens about once a day that a tab gets accidentally dragged to much around so that it spawns in a new window. The only solution then is to copy the url, close the new web browser window, open a new tab and load the url again in the main browser window. It is not even possible to undo closing the tab in the main browser window as its history will be completely moved to the new browser window as well.

Please leave a comment if you know of a way to disable the spawning of new Firefox windows when dragging and dropping tabs.