[kijin]

A very large percentage of Mozilla's revenue comes from search engines (recently Yahoo, previously Google) who pay Mozilla to make themselves the default search engine on Firefox. If Firefox users saw no ads and were untrackable, Yahoo would have no reason to pay anymore.

Of course Mozilla doesn't try to force everyone to stick to the defaults, so you're free to change the default search engine and install a bunch of ad-blocking, anti-tracking add-ons.