This is a very sensible thing on part of Mozilla. Community needs an open source browser to exist and thrive for number of reasons and hence Mozilla needs to figure out different ways to make money.

Mozilla is one brand I really trust for various reason. I am willing to rely more on Mozilla than on say Google for all my online existence.

One good business model for Mozilla could be to provide a subscription version of browser which is tightly integrated with more services of Mozilla including Webmail, Calendar, Photo and File Storage. Mozilla could charge me a monthly sum + show me ads on these services. Currently I use Zoho for many such services.