Firefox was launched at a time when web browsing was dominated by IE. [0] It is open-source in a world where browsers are either completely (IE/Edge) or somewhat (Chrome) closed source.
People say that IE didn't adapt fast enough, and that's why it died. But imagine how behind we'd be today if Firefox had never come along!
In a world where almost every piece of software you run (from your OS to your favorite toolbar) and every site you visit, tries to gather as much information about you. Mozilla's open-source stance and mission to improve privacy is protecting the web, by protecting us.
Perhaps privacy is not your favorite feature, perhaps it does not enhance your user experience, but for many others it does. It serves an important niche, and I hope that it succeeds and continues to disrupt what could otherwise be a browser monopoly.
[0] http://www.w3schools.com/browsers/