1) Open enough tabs force them to change width (compress horizontally). 2) attempt to close several tabs quickly. 3) get frustrated as the tabs automatically re-size to fill all the available horizontal space, moving the closing X button out of the way of the mouse, as opposed to the remaining tabs simply converging left, placing the next tab's X button where the previous one was. Firefox behaved this way in 11.x and now suddenly it doesn't. Slightly annoying...