This feature makes the drag and dropping of tabs into a new window much quicker and much more responsive.
This is a feature that's also in Google Chrome and Microsoft Edge. This will also not make the users unclick the mouse to make tabs into a new window. Please see the video below for more information about this idea.
The lack of this feature is the reason I stopped using firefox in the first place.
It seems like such a small thing but it makes a huge difference in user experience.