Firefox might be expecting a defined width in order to wrap it.
You need to use width and display properties together with word-wrap property:
width: 100px;
word-wrap: break-word;
display:inline-block;
Works both in IE & FF.
If you find this not working, use Firebug and see what CSS is being applied to the field you are trying to wrap. Check if the width & word-wrap properties are defined for it.