Using the drag and drop in the course editor with Firefox can lead to go back to a previous tab. If the insert line and an course element are both selected, which is only possible with Firefox, the browser will send 2 drop events. The first trigger an effective drop, the second is not recognize by the server and trigger the back button on the server.
- Update query and query UI to see if it's fix the issue
- If above is not sufficient, fix the issue