It's better to use a different attribute than other browsers until it becomes a standard. That way, you don't get unexpected behavior (READ: IE6 box model bugs

It's better to use a different attribute than other browsers until it becomes a standard. That way, you don't get unexpected behavior (READ: IE6 box model bugs that annoyed so many people) when it is finally standardized. Most of CSS3 has a ways to go until it becomes a standard, so Mozilla uses its own attribute to reflect its unique implementation/interpretation of what is there currently.