<div flex="50">
  <p>looooooooooooooong content</p>
</div>

When 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.