<div flex="50"> <p>looooooooooooooong content</p> </div>
white-space: nowrap; is used, simply putting
overflow: hidden; on <p> is not going to stop it from conquering the parent div and overflowing all over the place. Even if the div has a
flex-basis: 50%; to protect itself. That div is going to need a
overflow: hidden; too.
Nice job, Firefox 34.0.5.