I recently switched back to Firefox after years of using Chrome. Previously, the few times I had to fire up Firefox for development purposes it seemed sluggish to me. Not anymore.

Firefox has been super responsive to me, using less memory than the same browsing activity would on Chrome & Firefox-only (afaik) add-ons like Self Destructing Cookies are A+. On Android, Firefox Mobile works just as good or better than Chrome.

From a developer standpoint, Mozilla's recent work on the Web Extensions API (which is an almost complete match with Chrome's) is outstanding.