Image to illustrate, "+24 tabs" is to help recognize needed window
I use at least 2 Firefox windows for work, those windows are placed on different monitors and different virtual desktops for ease of access, and kind of have different purposes (I suppose other people can group tabs using windows too).
The problem is that sometimes I need to move tabs from one window to another. Detaching a tab, opening multitasking view, moving tab to a different desktop and then attach it to the second window isn't the easiest thing.
There is an extension that helps with this - 'Tab Mover', but Firefox already has Move Tab menu, so I think that aside from moving tabs to start/end it could also offer to move them between existing Firefox windows, especially when it already offers to move tabs to a new window.