The FF tab bar is so small that it's impossible to grab the window and move it, like to look at something hidden underneath it or make room for another window or app or whatever. The only places I can see to grab the window without accidentally opening tabs or dragging the tab off the tab bar into a new window are at each far end of the tab bar. So you have to decide which way you want to move the window and choose the appropriate end of the tab bar. Compared to Chrome, the FF tab bar is really skinny - even though the difference is only about 1/16 or 1/8 inch, that small difference is enough in Chrome to grab the window where you want and drag it without opening the tab right below where you're grabbing. I hope this description makes sense - hard to put it into words.