Firefox is open source. You don't have to have personally looked at its source or know anyone who works on it to trust that it's not recording your information, thousands of people have looked at its source, and if even one of them noticed something suspicious, the entire world would know about it.

Javelin isn't open source, so you're right, you have to have some amount of faith in an unknown group of developers.

Personally, that's not what's keeping me from using Javelin, I just need firefox sync support to consider it.