Firefox is and for the forseeable future will continue to be the best browser for those concerned with privacy. It may be marginalized, but it won't go away. With Electrolysis coming along nicely and Servo on the distant horizon, Firefox can probably also retake the crown as undisputed most secure browser at some point. They're also doing a decent job of keeping performance competitive.

Overall, Firefox seems about as healthy as you could expect given that it's competing against Microsoft, Apple, and Google.