Performance, portability, openness aside (there are many contenders here today), the main reason I use Firefox is because guys at Mozilla Foundation *seem* to care about my privacy *a bit* more than others. Or rather, they haven't designed Firefox from ground up to suck as much information about me as they can get away with.
Unfortunately, even though the potential is clearly here, Firefox does very little to actively protect my privacy. All the killer privacy features are pushed out to extensions. In 2015 there is no excuse for not shipping Adblock as a built-in component. I would really love to see filters being maintained and distributed within Mozilla - if nothing else, that would be a great way to engage the community.
Another extension which is "a must" for me, and badly suffers from integration issues, is Multifox. It lets me open several windows, each presenting a different identity to web servers. I believe it was designed to allow multiple simultaneous logins to services like Gmail but it has a nice side effect of being the most effective way of blocking trackers. They can get all the information about Youtube videos "I" watch, or what online banking "I" use, but they cannot easily connect these patches of information into a single consistent picture of "me". I bet such function will never make its way into Chrome or Safari, yet Mozilla chose to ignore the potential killer feature again.
So, why many of such essential privacy features are still not part of Firefox? I used to think it was because of Google founding but times have changed and Mozilla still does very little on that front (no, DNT really doesn't count).