The (blue) breakpoints that are not supposed to move can in fact be moved. To make this happen do the following:
Use the mouse to go to either breakpoint and make sure that the point is selected (with the black hole in the middle).
Now, use either the left arrow or right arrow key on the keyboard by holding one of them down for two seconds. You will see that the breakpoint moves and the 640 (or 1024) px size indeed changes as well. On occasion FF will crash when doing this. Once you exit FF and restart, the breakpoints that were moved are no longer blue and behave as though they are breakpoints set by the designer.