The old one is on the left, the new one on the right.

Yes, subtle. The fox has been toned down, given more feathers in the fur, and the world has changed.

Mozilla User Experience Designer Alex Faaborg explained the goal in May (Creative Brief for the New Firefox Icon):

Things we are planning to modernize 1) Shape of the tail (no longer 2D) 2) Textures (soft fox, glossy earth) 3) Simplify the overall level of detail

Much more and icon sets at Alex's blog: The New Firefox Icon.