a) The fact that they didn't appear in Snowden's documents like the other companies,
b) the fact that companies like Google (and Facebook, Yahoo, Microsoft, etc.) have it as their mission to build as precise user profiles as possible ("data hoover"), for example because the rely on advertising revenue. (Google is a 1-trick-pony, advertising being its trick.)
For the most part, Mozilla concentrates on releasing software rather than storing copious amounts of data about users. (Perhaps their "startup" Persona service is an exception to this, but I doubt it.) NSA is more likely to target all that juicy user data that Google has. They might also seek to undermine the security of software, but Mozilla aren't going to make that easy for them.
No company working with the NSA can ever be trusted. That only leaves Mozilla.
Plus, are there any other companies that are not-for-profit, like Mozilla? Because this point gives them a huge credit.