First of all, you probably will get less total comp at Mozilla than you could elsewhere. You'll still be paid well compared to many developers working outside SF/NYC/etc, though.

Second, instead of stock you'll get a higher potential bonus. The bonus percentage increases based on your level.

Third, benefits are generous. Being able to work remotely was a particularly unique and valuable one for a long time.

Fourth, the class and scale of the problems Mozilla is solving are interesting and challenging, which is a motivation for some employees.

Fifth, the mission and the commitment to open source are a motivation to many employees as well.

(These are just my personal observations as a current employee)