Apart from the open sourceness of browsers, it's worth to consider the culture of the companies behind them. Speaking of Mozilla, a non-profit, they have always cared about user rights and privacy and stayed true to their ideals for more than 10 years now. Something you can't say about Google, which has been tainted with all imaginable privacy issues and keeps actively luring people to stay connected in Chrome, Google+, Chromebook - all for a better surveillance experience.