I don't buy that. They are making a choice either way, whether they enable or disable it by default. Just like they are making a choice about the hundreds of other options Firefox has. A few examples of how Mozilla thinks:
> JavaScript enabled by default. Why? Developers and designers can do some really awesome websites with it and we want users to have that experience.
> SSL enabled by default. Why? It improves your security.
> Do Not Track disabled by default. Why? Because we don't want to fuck with the people who are paying us (i.e. Google).