When feasible, free and open source software like Firefox should be preferred to its proprietary counterparts; this helps to guarantee reliability, forward compatibility, and most of all, freedom.
Furthermore, while companies like Microsoft, Apple, and Google (who all make popular browsers) have made important contributions to computing, frequently they are compelled to sacrifice usability and interoperability for profit. While some of these browsers are nice to use (and even may presently be faster or more pleasant than Firefox) ultimately their purpose is to do what's best for the company. Very often, this includes functionality that is contrary to the best interests of the user.
The mission of the Mozilla Foundation is different; it is a non-profit dedicated to creating the best software experience for all users, and has done an exemplary job of this.